Fix bootstrap failure with the newest gnulib

[Bruno Haible]

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

0001-build-Fix-bootstrap-failure-with-the-newest-gnulib.patch
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