Fix:
conftest.c:26:7: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2006-05-01 12:14:51.000000000 -0500
+++ configure	2021-07-29 11:48:19.000000000 -0500
@@ -2892,6 +2892,7 @@
 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))