[3.12] gh-123917: Fix crypt check in configure (#123952)

Use a global volatile variable and check if the function is not NULL
to use the variable. Otherwise, a compiler optimization can remove
the variable making the check useless.

Co-authored-by: Paul Smith <paul@mad-scientist.net>

Author: vstinner

Misc/NEWS.d/next/Build/2024-09-11-16-06-42.gh-issue-123917.JuZl0r.rst

@@ -0,0 +1,2 @@
+Fix the check for the ``crypt()`` function in the configure script. Patch by
+Paul Smith and Victor Stinner.

configure

@@ -5237,12 +5237,14 @@ WITH_SAVE_ENV([
           #include <crypt.h>
         #endif
         #include <unistd.h>
+        volatile void *func;
       ], [
         #ifdef HAVE_CRYPT_R
-          void *x = crypt_r;
+          func = crypt_r;
         #else
-          void *x = crypt;
+          func = crypt;
         #endif
+          return (func != NULL);
       ])
     ], [ac_cv_crypt_crypt=yes], [ac_cv_crypt_crypt=no])
   ])