Add source tarballs to release process

As git provides a way to export a tarball given a commit, it's very easy to add the creation of source releases, IMHO. We very much like people to build from git tags, but apparently this is hard or at least inconvenient for some infrastructure out there. I hope the overhead is minimal enough for those doing the release...

This also disables the use of git-describe in exported source archives, as it results in a spurious error otherwise.

I hope this closes #2476.