Remove version.mk and references to it in the repo.

genversion.bat will now generate the same version strings as
update_version script, i.e. without 'v' prefix and without a
fallback if git archive is used.

If run from a release tarball it'll use package_version,
like configure. If run from a git repo, it will use the same
'git describe' with '-dirty' as update_version.

Signed-off-by: Ralph Giles <giles@thaumas.net>