Fix implicit declaration of function.
--- configure.orig	2006-03-13 22:59:56.000000000 -0600
+++ configure	2021-05-11 19:26:18.000000000 -0500
@@ -3498,6 +3498,7 @@
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 #include <ctype.h>
+#include <stdlib.h>
 #if ((' ' & 0x0FF) == 0x020)
 # define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 # define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))