Rename version.m4 to autoconf_version.m4.

Rename version.m4 to autoconf_version.m4.

Many existing projects have their own version.m4, and in some cases
having one is recommended by the Automake manual.  As of 2.72, the
name collision causes a spurious warning about including ‘version.m4’
multiple times, because the “has this been included before” machinery
does not track full pathnames.

A proper fix is more complicated than anyone has time to develop;
let’s just rename our version.m4 to autoconf_version.m4, which should
be sufficiently distinctive.

Reported in <https://savannah.gnu.org/support/?111007>.

* lib/version.in: Rename to lib/autoconf_version.in.
* bootstrap, configure.ac, lib/freeze.mk, lib/local.mk,
  lib/m4sugar/m4sugar.m4: Change all uses of version.m4 to
  autoconf_version.m4, and all uses of version.in to
  autoconf_version.in.