git-version-gen: add --fallback option to use if git is not present

git-version-gen: add --fallback option to use if git is not present

When building in a git checkout, but from a system lacking git, it
is useful to fall back to the version determined when the git
checkout was last used from a system sporting git.

* build-aux/git-version-gen: Add support for the new option --fallback,
which comes into play when there is no $tarball_version_file and
git is not working.
(scriptversion): Update.

Copyright-paperwork-exempt: yes
Signed-off-by: Peter Rosin <peda@lysator.liu.se>