Fix:
conftest.c:91:4: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2016-10-23 17:34:16.000000000 -0500
+++ configure	2020-11-27 01:56:31.000000000 -0600
@@ -8307,6 +8307,7 @@
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);
@@ -8352,6 +8353,7 @@
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);
@@ -8397,6 +8399,7 @@
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);