Fix:

configure:1548:67: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]

--- configure.orig	2007-02-05 21:32:14.000000000 -0600
+++ configure	2021-02-14 00:44:47.000000000 -0600
@@ -1541,6 +1541,7 @@
 #line 1542 "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)))
@@ -1973,6 +1974,7 @@
 #line 1974 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");
@@ -2012,6 +2014,7 @@
 #line 2013 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");
@@ -2051,6 +2054,7 @@
 #line 2052 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");
@@ -2090,6 +2094,7 @@
 #line 2091 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");
@@ -2129,6 +2134,7 @@
 #line 2130 "configure"
 #include "confdefs.h"
 #include <stdio.h>
+#include <stdlib.h>
 main()
 {
   FILE *f=fopen("conftestval", "w");