Fix:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2003-11-14 17:16:29.000000000 -0600
+++ configure	2020-12-18 04:55:27.000000000 -0600
@@ -1217,6 +1217,7 @@
 #line 1218 "configure"
 #include "confdefs.h"
 #include <ctype.h>
+#include <stdlib.h>
 #define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 #define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))
 #define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
@@ -1324,6 +1325,7 @@
 #line 1325 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");
@@ -1363,6 +1365,7 @@
 #line 1364 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");
@@ -1402,6 +1405,7 @@
 #line 1403 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");