Fix implicit declarations

error: call to undeclared library function 'exit' with type
'void (int) __attribute__((noreturn))' [-Wimplicit-function-declaration]

See: https://trac.macports.org/wiki/WimplicitFunctionDeclaration

diff -urN a/configure b/configure
--- a/configure	2015-02-23 05:47:52.000000000 -0500
+++ b/configure	2025-08-26 21:31:22.000000000 -0400
@@ -3218,6 +3218,7 @@
 cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
+#include <stdlib.h>
 #include <ctype.h>
 #if ((' ' & 0x0FF) == 0x020)
 # define ISLOWER(c) ('a' <= (c) && (c) <= 'z')