patch gnulib to accomodate autoconf 2.70

Upgrading gnulib turned to be too difficult, so this only includes
updates to m4 files necessary to please Autoconf 2.70.

For issue #447.

* m4/extensions.m4, m4/malloc.m4, m4/std-gnu11.m4: Cherry-pick
from today's gnulib version.

m4/malloc.m4

@@ -1,5 +1,5 @@
-# malloc.m4 serial 20
-dnl Copyright (C) 2007, 2009-2020 Free Software Foundation, Inc.
+# malloc.m4 serial 21
+dnl Copyright (C) 2007, 2009-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
@@ -8,18 +8,12 @@ dnl with or without modifications, as long as this notice is preserved.
 # https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=04be2b7a29d65d9a08e64e8e56e594c91749598c
 AC_DEFUN([_AC_FUNC_MALLOC_IF],
 [
-  AC_REQUIRE([AC_HEADER_STDC])dnl
   AC_REQUIRE([AC_CANONICAL_HOST])dnl for cross-compiles
-  AC_CHECK_HEADERS([stdlib.h])
   AC_CACHE_CHECK([for GNU libc compatible malloc],
     [ac_cv_func_malloc_0_nonnull],
     [AC_RUN_IFELSE(
        [AC_LANG_PROGRAM(
-          [[#if defined STDC_HEADERS || defined HAVE_STDLIB_H
-            # include <stdlib.h>
-            #else
-            char *malloc ();
-            #endif
+          [[#include <stdlib.h>
           ]],
           [[char *p = malloc (0);
             int result = !p;