Embed version in git-archive tarballs and populate VERSION from it when set

[jas4711]

Here is an idea for this -- it works if the tarballs are created by 'git archive', for example like this:

git archive --prefix=ssh-tpm-agent-$(git describe --abbrev=7)/ -o ssh-tpm-agent-$(git describe --abbrev=7)-src.tar.gz HEAD

Some git hosting sites supports .gitattribute $Format$ export substitution, not sure about GitHub.

Cheers,
Simon

Closes: #90

[Foxboron]

Cool, thanks. I need to test this before merging :) I'll get on it at some point.