diff --git a/lisp/emacs-lisp/ert-x.el b/lisp/emacs-lisp/ert-x.el
index ec2106dda22..98e6b2cb1b6 100644
--- a/lisp/emacs-lisp/ert-x.el
+++ b/lisp/emacs-lisp/ert-x.el
@@ -374,7 +374,9 @@ The same keyword arguments are supported as in
 
 (defun ert-gcc-is-clang-p ()
   "Return non-nil if the `gcc' command actually runs the Clang compiler."
-  (internal--gcc-is-clang-p))
+  (require 'ffap)
+  (declare-function ffap--gcc-is-clang-p "ffap" ())
+  (ffap--gcc-is-clang-p))
 
 (defvar tramp-default-host-alist)
 (defvar tramp-methods)
diff --git a/lisp/ffap.el b/lisp/ffap.el
index 890a227fca9..f892bb81e40 100644
--- a/lisp/ffap.el
+++ b/lisp/ffap.el
@@ -179,12 +179,13 @@ Note this name may be omitted if it equals the default
   :group 'ffap)
 
 (defvar ffap-url-regexp
-  (concat
-   "\\("
-   "news\\(post\\)?:\\|mailto:\\|file:" ; no host ok
-   "\\|"
-   "\\(ftp\\|https?\\|telnet\\|gopher\\|gemini\\|www\\|wais\\)://" ; needs host
-   "\\)")
+  (eval-when-compile
+    (concat
+     "\\("
+     "news\\(post\\)?:\\|mailto:\\|file:" ; no host ok
+     "\\|"
+     "\\(ftp\\|https?\\|telnet\\|gopher\\|gemini\\|www\\|wais\\)://" ;Needs host
+     "\\)"))
   "Regexp matching the beginning of a URI, for ffap.
 If the value is nil, disable URL-matching features in ffap.")
 
@@ -831,13 +832,79 @@ to extract substrings.")
   (and (not (string-match "\\.el\\'" name))
        (ffap-locate-file name '(".el") load-path)))
 
-(defvar ffap-c-path (internal--c-header-file-path)
+(defun ffap--gcc-is-clang-p ()
+  "Return non-nil if the `gcc' command actually runs the Clang compiler."
+  ;; Recent macOS machines run llvm when you type gcc by default.  (!)
+  ;; We can't even check if it's a symlink; it's a binary placed in
+  ;; "/usr/bin/gcc".  So we need to check the output.
+  (when-let* ((out (ignore-errors
+                     (with-temp-buffer
+                       (call-process "gcc" nil t nil "--version")
+                       (buffer-string)))))
+   (string-match "Apple \\(LLVM\\|[Cc]lang\\)\\|Xcode\\.app" out)))
+
+(defun ffap--c-path ()
+  "Return search path for C header files (a list of strings)."
+  ;; FIXME: It's not clear that this is a good place to put this, or
+  ;; even that this should necessarily be internal.
+  ;; See also (Bug#10702):
+  ;; cc-search-directories, semantic-c-dependency-system-include-path,
+  ;; semantic-gcc-setup
+  (delete-dups
+   ;; We treat MS-Windows/MS-DOS specially, since there's no
+   ;; widely-accepted canonical directory for C include files.
+   (let ((base (if (not (memq system-type '(windows-nt ms-dos)))
+                   '("/usr/include" "/usr/local/include")))
+         (call-clang-p (or (ffap--gcc-is-clang-p)
+                           (and (executable-find "clang")
+                                (not (executable-find "gcc"))))))
+     (cond ((or call-clang-p
+                (memq system-type '(windows-nt ms-dos)))
+            ;; This is either macOS, or MS-Windows/MS-DOS, or a system
+            ;; with clang only.
+            (with-temp-buffer
+              (ignore-errors
+                (call-process (if call-clang-p "clang" "gcc")
+                              nil t nil
+                              "-v" "-E" "-"))
+              (goto-char (point-min))
+              (narrow-to-region
+               (save-excursion
+                 (re-search-forward
+                  "^#include <\\.\\.\\.> search starts here:\n" nil t)
+                 (point))
+               (save-excursion
+                 (re-search-forward "^End of search list.$" nil t)
+                 (pos-bol)))
+              (while (search-forward "(framework directory)" nil t)
+                (delete-line))
+              ;; "gcc -v" reports file names with many "..", so we
+              ;; normalize it.
+              (or (mapcar #'expand-file-name
+                          (append base
+                                  (split-string (buffer-substring-no-properties
+                                                 (point-min) (point-max)))))
+                  ;; Fallback for whedn the compiler is not available.
+                  (list (expand-file-name "/usr/include")
+                        (expand-file-name "/usr/local/include")))))
+           ;; Prefer GCC.
+           ((let ((arch (with-temp-buffer
+                          (when (eq 0 (ignore-errors
+                                        (call-process "gcc" nil '(t nil) nil
+                                                      "-print-multiarch")))
+                            (goto-char (point-min))
+                            (buffer-substring (point) (line-end-position))))))
+              (if (zerop (length arch))
+                  base
+                (append base (list (expand-file-name arch "/usr/include"))))))))))
+
+(defvar ffap-c-path (ffap--c-path)      ;FIXME: Delay initialization?
   "List of directories to search for include files.")
 
 (defun ffap-c-mode (name)
   (ffap-locate-file name t ffap-c-path))
 
-(defvar ffap-c++-path
+(defvar ffap-c++-path      ;FIXME: Delay initialization?
   (let ((c++-include-dir (with-temp-buffer
                            (when (eq 0 (ignore-errors
                                          (call-process "g++" nil t nil "-v")))
diff --git a/lisp/man.el b/lisp/man.el
index 681758cadf5..4d5e8e323ca 100644
--- a/lisp/man.el
+++ b/lisp/man.el
@@ -230,10 +230,11 @@ the associated section number."
   :type '(repeat (cons (string :tag "Bogus Section")
 		       (string :tag "Real Section"))))
 
-(defcustom Man-header-file-path (internal--c-header-file-path)
+(defcustom Man-header-file-path t
   "C Header file search path used in Man."
   :version "31.1"
-  :type '(repeat string))
+  :type '(choice (repeat string)
+                 (const :tag "Use 'ffap-c-path'" t)))
 
 (defcustom Man-name-local-regexp (concat "^" (regexp-opt '("NOM" "NAME")) "$")
   "Regexp that matches the text that precedes the command's name.
@@ -571,7 +572,11 @@ list of directories where the remote system has the C header files."
   (let ((remote-id (file-remote-p default-directory)))
     (if (null remote-id)
         ;; The local case.
-        Man-header-file-path
+        (if (not (eq t Man-header-file-path))
+            Man-header-file-path
+          (require 'ffap)
+          (defvar ffap-c-path)
+          ffap-c-path)
       ;; The remote case.  Use connection-local variables.
       (mapcar
        (lambda (elt) (concat remote-id elt))
diff --git a/lisp/subr.el b/lisp/subr.el
index c7be73224cf..2395d1bd85a 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -7627,70 +7627,4 @@ and return the value found in PLACE instead."
                ,(funcall setter val)
                ,val)))))
 
-(defun internal--gcc-is-clang-p ()
-  "Return non-nil if the `gcc' command actually runs the Clang compiler."
-  ;; Recent macOS machines run llvm when you type gcc by default.  (!)
-  ;; We can't even check if it's a symlink; it's a binary placed in
-  ;; "/usr/bin/gcc".  So we need to check the output.
-  (when-let* ((out (ignore-errors
-                     (with-temp-buffer
-                       (call-process "gcc" nil t nil "--version")
-                       (buffer-string)))))
-    (string-match "Apple \\(LLVM\\|[Cc]lang\\)\\|Xcode\\.app" out)))
-
-(defun internal--c-header-file-path ()
-  "Return search path for C header files (a list of strings)."
-  ;; FIXME: It's not clear that this is a good place to put this, or
-  ;; even that this should necessarily be internal.
-  ;; See also (Bug#10702):
-  ;; cc-search-directories, semantic-c-dependency-system-include-path,
-  ;; semantic-gcc-setup
-  (delete-dups
-   ;; We treat MS-Windows/MS-DOS specially, since there's no
-   ;; widely-accepted canonical directory for C include files.
-   (let ((base (if (not (memq system-type '(windows-nt ms-dos)))
-                   '("/usr/include" "/usr/local/include")))
-         (call-clang-p (or (internal--gcc-is-clang-p)
-                           (and (executable-find "clang")
-                                (not (executable-find "gcc"))))))
-     (cond ((or call-clang-p
-                (memq system-type '(windows-nt ms-dos)))
-            ;; This is either macOS, or MS-Windows/MS-DOS, or a system
-            ;; with clang only.
-            (with-temp-buffer
-              (ignore-errors
-                (call-process (if call-clang-p "clang" "gcc")
-                              nil t nil
-                              "-v" "-E" "-"))
-              (goto-char (point-min))
-              (narrow-to-region
-               (save-excursion
-                 (re-search-forward
-                  "^#include <\\.\\.\\.> search starts here:\n" nil t)
-                 (point))
-               (save-excursion
-                 (re-search-forward "^End of search list.$" nil t)
-                 (pos-bol)))
-              (while (search-forward "(framework directory)" nil t)
-                (delete-line))
-              ;; "gcc -v" reports file names with many "..", so we
-              ;; normalize it.
-              (or (mapcar #'expand-file-name
-                          (append base
-                                  (split-string (buffer-substring-no-properties
-                                                 (point-min) (point-max)))))
-                  ;; Fallback for whedn the compiler is not available.
-                  (list (expand-file-name "/usr/include")
-                        (expand-file-name "/usr/local/include")))))
-           ;; Prefer GCC.
-           ((let ((arch (with-temp-buffer
-                          (when (eq 0 (ignore-errors
-                                        (call-process "gcc" nil '(t nil) nil
-                                                      "-print-multiarch")))
-                            (goto-char (point-min))
-                            (buffer-substring (point) (line-end-position))))))
-              (if (zerop (length arch))
-                  base
-                (append base (list (expand-file-name arch "/usr/include"))))))))))
-
 ;;; subr.el ends here
diff --git a/test/lisp/ffap-tests.el b/test/lisp/ffap-tests.el
index 9bd78ae0425..b07a4c4fbc6 100644
--- a/test/lisp/ffap-tests.el
+++ b/test/lisp/ffap-tests.el
@@ -182,6 +182,71 @@ left alone when opening a URL in an external browser."
     (goto-char (point-min))
     (should (equal (ffap-file-at-point) nil))))
 
+(ert-deftest ffap-tests--c-path ()
+  (should (seq-every-p #'stringp (ffap--c-path)))
+  (should (locate-file "stdio.h" (ffap--c-path)))
+  (or (memq system-type '(windows-nt ms-dos))
+      (should (member "/usr/include" (ffap--c-path))))
+  (should (equal (ffap--c-path)
+                 (delete-dups (ffap--c-path))))
+  ;; Return a meaningful result even if calling some compiler fails.
+  (cl-letf (((symbol-function 'call-process)
+             (lambda (_program &optional _infile _destination _display &rest _args) 1)))
+    (should (seq-every-p #'stringp (ffap--c-path)))
+    (should (member (expand-file-name "/usr/include")
+                    (ffap--c-path)))
+    (should (equal (ffap--c-path)
+                   (delete-dups (ffap--c-path))))))
+
+(ert-deftest ffap-tests--c-path/gcc-mocked ()
+  ;; Handle empty values of "gcc -print-multiarch".
+  (cl-letf (((symbol-function 'call-process)
+             (lambda (_program &optional _infile _destination _display &rest args)
+               (when (equal (car args) "-print-multiarch")
+                 (insert "\n") 0))))
+    (should (member (expand-file-name "/usr/include")
+                    (ffap--c-path))))
+  ;; Handle single values of "gcc -print-multiarch".
+  (cl-letf ((system-type 'foo)
+            ((symbol-function 'call-process)
+             (lambda (_program &optional _infile _destination _display &rest args)
+               (when (equal (car args) "-print-multiarch")
+                 (insert "x86_64-linux-gnu\n") 0))))
+    (should (member (expand-file-name "/usr/include/x86_64-linux-gnu")
+                    (ffap--c-path)))))
+
+(ert-deftest ffap-tests--c-path/clang-mocked ()
+  ;; Handle clang 15.0.0 output on macOS 15.2.
+  (cl-letf (((symbol-function 'ffap--gcc-is-clang-p) (lambda () t))
+            ((symbol-function 'call-process)
+             (lambda (_program &optional _infile _destination _display &rest _args)
+               (insert "\
+Apple clang version 15.0.0 (clang-1500.3.9.4)
+Target: arm64-apple-darwin24.2.0
+Thread model: posix
+InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
+ \"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang\"
+[[[...Emacs test omits some verbose junk from the output here...]]]
+clang -cc1 version 15.0.0 (clang-1500.3.9.4) default target arm64-apple-darwin24.2.0
+ignoring nonexistent directory \"/usr/local/include\"
+#include \"...\" search starts here:
+#include <...> search starts here:
+ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include
+ /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
+ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
+ /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
+End of search list.
+# 1 \"<stdin>\"
+# 1 \"<built-in>\" 1
+# 1 \"<built-in>\" 3
+# 418 \"<built-in>\" 3
+# 1 \"<command line>\" 1
+# 1 \"<built-in>\" 2
+# 1 \"<stdin>\" 2")
+               0)))
+    (should (member (expand-file-name "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include")
+                    (ffap--c-path)))))
+
 (provide 'ffap-tests)
 
 ;;; ffap-tests.el ends here
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index c2f64867d90..98fd1cad894 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -1463,70 +1463,5 @@ final or penultimate step during initialization."))
                  (props-out (object-intervals out)))
             (should (equal props-out props-in))))))))
 
-(ert-deftest subr-tests-internal--c-header-file-path ()
-  (should (seq-every-p #'stringp (internal--c-header-file-path)))
-  (should (locate-file "stdio.h" (internal--c-header-file-path)))
-  (or (memq system-type '(windows-nt ms-dos))
-      (should (member "/usr/include" (internal--c-header-file-path))))
-  (should (equal (internal--c-header-file-path)
-                 (delete-dups (internal--c-header-file-path))))
-  ;; Return a meaningful result even if calling some compiler fails.
-  (cl-letf (((symbol-function 'call-process)
-             (lambda (_program &optional _infile _destination _display &rest _args) 1)))
-    (should (seq-every-p #'stringp (internal--c-header-file-path)))
-    (should (member (expand-file-name "/usr/include")
-                    (internal--c-header-file-path)))
-    (should (equal (internal--c-header-file-path)
-                   (delete-dups (internal--c-header-file-path))))))
-
-(ert-deftest subr-tests-internal--c-header-file-path/gcc-mocked ()
-  ;; Handle empty values of "gcc -print-multiarch".
-  (cl-letf (((symbol-function 'call-process)
-             (lambda (_program &optional _infile _destination _display &rest args)
-               (when (equal (car args) "-print-multiarch")
-                 (insert "\n") 0))))
-    (should (member (expand-file-name "/usr/include")
-                    (internal--c-header-file-path))))
-  ;; Handle single values of "gcc -print-multiarch".
-  (cl-letf ((system-type 'foo)
-            ((symbol-function 'call-process)
-             (lambda (_program &optional _infile _destination _display &rest args)
-               (when (equal (car args) "-print-multiarch")
-                 (insert "x86_64-linux-gnu\n") 0))))
-    (should (member (expand-file-name "/usr/include/x86_64-linux-gnu")
-                    (internal--c-header-file-path)))))
-
-(ert-deftest subr-tests-internal--c-header-file-path/clang-mocked ()
-  ;; Handle clang 15.0.0 output on macOS 15.2.
-  (cl-letf (((symbol-function 'internal--gcc-is-clang-p) (lambda () t))
-            ((symbol-function 'call-process)
-             (lambda (_program &optional _infile _destination _display &rest _args)
-               (insert "\
-Apple clang version 15.0.0 (clang-1500.3.9.4)
-Target: arm64-apple-darwin24.2.0
-Thread model: posix
-InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
- \"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang\"
-[[[...Emacs test omits some verbose junk from the output here...]]]
-clang -cc1 version 15.0.0 (clang-1500.3.9.4) default target arm64-apple-darwin24.2.0
-ignoring nonexistent directory \"/usr/local/include\"
-#include \"...\" search starts here:
-#include <...> search starts here:
- /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include
- /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
- /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
- /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
-End of search list.
-# 1 \"<stdin>\"
-# 1 \"<built-in>\" 1
-# 1 \"<built-in>\" 3
-# 418 \"<built-in>\" 3
-# 1 \"<command line>\" 1
-# 1 \"<built-in>\" 2
-# 1 \"<stdin>\" 2")
-               0)))
-    (should (member (expand-file-name "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include")
-                    (internal--c-header-file-path)))))
-
 (provide 'subr-tests)
 ;;; subr-tests.el ends here