Fix implicit function 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	2003-06-18 09:07:26.000000000 -0400
+++ b/configure	2025-10-10 19:58:06.968105588 -0400
@@ -1154,6 +1154,7 @@
   cat > conftest.$ac_ext <<EOF
 #line 1156 "configure"
 #include "confdefs.h"
+#include <stdlib.h>
 #include <ctype.h>
 #define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 #define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))