Fix:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2009-07-09 17:03:57.000000000 -0500
+++ configure	2021-01-23 01:47:41.000000000 -0600
@@ -4337,6 +4337,7 @@
 cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
+#include <stdlib.h>
 #include <sys/stat.h>
 int
 main ()
@@ -6275,6 +6276,7 @@
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 
+#include <stdlib.h>
 #include <sys/types.h>
 #include <regex.h>
 main() {