Fix:

configure:1354:67: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
getopt.c:577:30: error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]
getopt.c:653:7: error: implicitly declaring library function 'strncmp' with type 'int (const char *, const char *, unsigned long)' [-Werror,-Wimplicit-function-declaration]
getopt.c:656:21: error: implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2001-05-03 11:44:22.000000000 -0500
+++ configure	2021-06-20 21:30:37.000000000 -0500
@@ -1347,6 +1347,7 @@
 #line 1348 "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)))
--- lib/getopt.c.orig	1997-04-10 04:42:52.000000000 -0500
+++ lib/getopt.c	2021-06-20 23:35:45.000000000 -0500
@@ -43,6 +43,7 @@
 #endif
 
 #include <stdio.h>
+#include <string.h>
 
 /* Comment out all this code if we are using the GNU C Library, and are not
    actually compiling the library itself.  This code is part of the GNU C