Fix bootstrap failure with the newest gnulib

With the newest gnulib, the Continuous Integration fails already in the
'bootstrap' phase [1]:

bootstrap: running: /builds/gnu-m4/ci-distcheck/gnulib/gnulib-tool --no-changelog --no-libtool --symlink --update
/builds/gnu-m4/ci-distcheck/gnulib/gnulib-tool: *** minimum supported autoconf version is 2.64. Try adding AC_PREREQ([2.64]) to your configure.ac.
/builds/gnu-m4/ci-distcheck/gnulib/gnulib-tool: *** Stop.

I can reproduce it locally:

$ ./bootstrap --skip-git --gnulib-srcdir=/GNULIB
...
bootstrap: running: /GNULIB/gnulib-tool --no-changelog --no-libtool --symlink --update
/GNULIB/gnulib-tool: *** minimum supported autoconf version is 2.64. Try adding AC_PREREQ([2.64]) to your configure.ac.
/GNULIB/gnulib-tool: *** Stop.


The attached patch fixes it.


[1] https://gitlab.com/gnu-m4/ci-distcheck/-/jobs/698627302/raw

From df29c5b764b540940e764a92400a7e6907d7fce3 Mon Sep 17 00:00:00 2001
From: Bruno Haible <[email protected]>
Date: Sun, 23 Aug 2020 01:33:08 +0200
Subject: [PATCH] build: Fix bootstrap failure with the newest gnulib.

* configure.ac: Require Autoconf 2.64 at least.
---
 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index c648dd7..888a08a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-AC_PREREQ([2.63])
+AC_PREREQ([2.64])
 AC_INIT([GNU M4], m4_esyscmd([build-aux/git-version-gen .tarball-version]),
   [[email protected]])
 AC_CONFIG_AUX_DIR([build-aux])
-- 
2.7.4