Fix:

conftest.c:51:7: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]

getopt.c:576:30: error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]

getopt.c:655:21: error: implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Werror,-Wimplicit-function-declaration]

utils.c:96:4: error: implicit declaration of function 'unlink' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

sed.c:178:3: error: implicit declaration of function 'initialize_mbcs' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

--- configure.orig	2005-02-08 11:31:24.000000000 -0600
+++ configure	2021-02-14 00:09:59.000000000 -0600
@@ -3376,8 +3376,8 @@
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -5738,7 +5738,7 @@
       if (!in)
 	return 1;
       len = getline (&line, &siz, in);
-      exit ((len == 4 && line && strcmp (line, "foo\n") == 0) ? 0 : 1);
+      return (len == 4 && line && strcmp (line, "foo\n") == 0) ? 0 : 1;
     }
 
 _ACEOF
--- lib/getopt.c.orig	2004-11-06 05:37:29.000000000 -0600
+++ lib/getopt.c	2021-02-14 00:18:13.000000000 -0600
@@ -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
--- lib/utils.c.orig	2004-11-06 05:37:29.000000000 -0600
+++ lib/utils.c	2021-02-14 00:13:38.000000000 -0600
@@ -35,6 +35,10 @@
 # include <stdlib.h>
 #endif /* HAVE_STDLIB_H */
 
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
+
 #include "utils.h"
 
 const char *myname;
--- sed/sed.c.orig	2004-11-11 08:33:59.000000000 -0600
+++ sed/sed.c	2021-02-14 00:23:06.000000000 -0600
@@ -59,6 +59,8 @@
 # define ATOI(x)	strtoul(x, NULL, 0)
 #endif
 
+extern void initialize_mbcs();
+
 int extended_regexp_flags = 0;
 
 /* If set, fflush(stdout) on every line output. */