Use git describe to populate autoconf's VERSION.

This uses the output of "git describe" directly in configure.ac to
determine the version number to use, instead of hardcoding it.

With this final change, current version information is completely
removed from the codebase itself, and is always fetched on-the-fly
from git as the single source of truth.

On release, an automated process ensures that the current commit is
tagged, regenerates the configure script to make sure it has the final
version information from git (this is important as it is then used in
the tinc version output code itself if building from a release tarball
because git is unavailable then), and the normal autoconf check makes
sure that the tag appears in NEWS.

Note that the prompt to edit NEWS is removed, because the new process
requires the release to be tagged beforehand, and NEWS must be committed
before the tag is applied.