readline: use upstream's aclocal.m4 as acinclude.m4

Instead of shipping a fork of the upstream aclocal.m4, simply rename it to
acinclude.m4 at configure time.  We don't need the fork now that autoheader is
excluded.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>