announce-gen: emit Gnulib's git-based version string.

announce-gen: emit Gnulib's git-based version string.

* build-aux/announce-gen: Remove option: --gnulib-snapshot-time-stamp=S.
New option --gnulib-version=V, where V is expected to be
the output of running git describe in the gnulib directory.
(get_tool_versions): Request feedback on xdelta.  I suspect it's
not useful, and plan to stop publishing an xdelta file with each
coreutils release.