Update Gnulib to v0.1-4379-g2ef5a9b4b

Also bump required autoconf version to 2.64, as required by Gnulib.

lib/mempcpy.c

@@ -0,0 +1,28 @@
+/* Copy memory area and return pointer after last written byte.
+   Copyright (C) 2003, 2007, 2009-2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <string.h>
+
+/* Copy N bytes of SRC to DEST, return pointer to bytes after the
+   last written byte.  */
+void *
+mempcpy (void *dest, const void *src, size_t n)
+{
+  return (char *) memcpy (dest, src, n) + n;
+}