Fix:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2006-02-21 02:03:22.000000000 -0600
+++ configure	2021-07-06 17:56:07.000000000 -0500
@@ -5252,6 +5252,7 @@
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 #include <ctype.h>
+#include <stdlib.h>
 #if ((' ' & 0x0FF) == 0x020)
 # define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 # define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))
@@ -27156,6 +27157,7 @@
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 
+#include <stdlib.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/time.h>
@@ -27232,6 +27234,7 @@
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 
+#include <stdlib.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/time.h>
