Fix autoconf 2.59 bug that results in an incorrect definition of
STDC_HEADERS with some compilers.

Raised upstream: <https://bz.apache.org/SpamAssassin/show_bug.cgi?id=8204>

diff -ur Mail-SpamAssassin-4.0.0.orig/spamc/configure Mail-SpamAssassin-4.0.0/spamc/configure
--- Mail-SpamAssassin-4.0.0.orig/spamc/configure	2022-12-14 07:03:27.000000000 +0100
+++ Mail-SpamAssassin-4.0.0/spamc/configure	2023-12-19 14:34:05.414644959 +0100
@@ -2119,7 +2119,7 @@
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -2624,8 +2624,8 @@
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext