Re-fix build warnings about subr-x defsubsts

* lisp/term/haiku-win.el (require):
* lisp/progmodes/elisp-mode.el (require):
* lisp/isearch.el (require): Require subr-x at compile time, since
these use defsubsts/macros from there.

* lisp/emacs-lisp/subr-x.el (string-empty-p): Move from here...
* lisp/simple.el (string-empty-p): ... to here.  This is to help
with a build problem where files.el is using the defsubst, but
requiring subr-x.el at compile time leads to load errors.

lisp/emacs-lisp/subr-x.el

@@ -97,10 +97,6 @@ threading."
     (maphash (lambda (_ v) (push v values)) hash-table)
     values))
 
-(defsubst string-empty-p (string)
-  "Check whether STRING is empty."
-  (string= string ""))
-
 (defsubst string-join (strings &optional separator)
   "Join all STRINGS using SEPARATOR.
 Optional argument SEPARATOR must be a string, a vector, or a list of

lisp/isearch.el

@@ -54,6 +54,7 @@
 ;;; Code:
 
 (eval-when-compile (require 'cl-lib))
+(eval-when-compile (require 'subr-x))
 
 ;; Some additional options and constants.
 

lisp/progmodes/elisp-mode.el

@@ -31,6 +31,7 @@
 (require 'cl-generic)
 (require 'lisp-mode)
 (eval-when-compile (require 'cl-lib))
+(eval-when-compile (require 'subr-x))
 
 (define-abbrev-table 'emacs-lisp-mode-abbrev-table ()
   "Abbrev table for Emacs Lisp mode.

lisp/simple.el

@@ -10620,6 +10620,10 @@ If the buffer doesn't exist, create it first."
           (save-buffer)))
       t)))
 
+(defsubst string-empty-p (string)
+  "Check whether STRING is empty."
+  (string= string ""))
+
 
 
 (provide 'simple)

lisp/term/haiku-win.el

@@ -24,6 +24,7 @@
 ;;; Code:
 
 (eval-when-compile (require 'cl-lib))
+(eval-when-compile (require 'subr-x))
 (unless (featurep 'haiku)
   (error "%s: Loading haiku-win without having Haiku"
          invocation-name))