build: use the gnulib version of the .m4 files when present

build: use the gnulib version of the .m4 files when present

prevent aclocal from preferring .m4 files under m4/ over the version
provided by gnulib, by using only one directory.

I have noticed this after './configure --help' gave me two different
versions of "--enable-threads".  This was caused by aclocal that
preferred the version of lock.m4 provided by autopoint instead of
using the newer version distributed with gnulib.

Having two different directories made sense back when we checked
gnulib files into libvirt.git, but that was ages ago.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>