Makefile: Add an explicit version file to the repository.

We do this so that "git archive" produces a usable tar file without us
having to post-modify it, (since tools like git-buildpackage might not
give us an easy way to hook into the tar-file-creation step).

To support this we also have to change our preference to prefer the
git-described-based version (if available) and only if not available
do we fallback to using what's in the "version" file. Finally, we also
ovverride this preference when releasing, (where what's in the
"version" file wins).

Note that using our Makefile's rule to create a tar file still will
insert the git-based version into the tar file. This is useful for
creating snapshots which will correctly report the git version from
which they were created.