Factor out function to check for clang in tests

This also stops a flymake test from failing on my machine.

* lisp/emacs-lisp/ert-x.el (ert-gcc-is-clang-p): New function
factored out from ...
* test/lisp/progmodes/flymake-tests.el (flymake-tests--gcc-is-clang):
* test/lisp/cedet/semantic/bovine/gcc-tests.el
(semantic-gcc-test-output-parser-this-machine): ... here.

* test/lisp/progmodes/flymake-tests.el (different-diagnostic-types)
(included-c-header-files):
* test/lisp/cedet/semantic/bovine/gcc-tests.el
(semantic-gcc-test-output-parser-this-machine): Use above new function.

lisp/emacs-lisp/ert-x.el

@@ -475,6 +475,14 @@ The same keyword arguments are supported as in
      :directory t
      ,@body))
 
+(defun ert-gcc-is-clang-p ()
+  "Return non-nil if the `gcc' command actually runs the Clang compiler."
+  ;; Some macOS machines run llvm when you type gcc.  (!)
+  ;; 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.
+  (string-match "Apple \\(LLVM\\|[Cc]lang\\)\\|Xcode\\.app"
+                (shell-command-to-string "gcc --version")))
+
 (provide 'ert-x)
 
 ;;; ert-x.el ends here