gh-123917: Fix crypt check in configure

Check if the function is not NULL to use the variable. Otherwise, a
compiler can remove the variable making the check useless.

Co-Authored-By: Paul Smith <[email protected]>

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

@@ -5243,6 +5243,7 @@ WITH_SAVE_ENV([
         #else
           void *x = crypt;
         #endif
+          return (x != NULL);
       ])
     ], [ac_cv_crypt_crypt=yes], [ac_cv_crypt_crypt=no])
   ])