RFC: use git-version-gen from gnulib

Hi.

I'd like to propose to switch from local (unmaintained?) copy of git-version-gen script and use it directly from upstream gnulib package. It's available pretty-much everywhere (including ancient Debian which we support for nightly). It's also available in meta-oe: https://layers.openembedded.org/layerindex/recipe/47365/

It doesn't feel right to use copy-pasted code when we could offload the maintenance work to packaging/dependency tools.

What do you think? Please contribute to https://osmocom.org/issues/2467 which I've created to track this RFC.