Ignore tarballs too.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Makefile.am: Make it possible to append to VERSION

When building in a CI environment, that preserves generated tarballs
too, make it possible to append SNAPSHOT_VERSION to VERSION. This does
not change the version reported by the library, but we can get a
timestamped tarball name.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>