* cmake: fix handling the case where `git describe` fails

* cmake: fix version recorded in releases

If downloaded as a tarball release, there will be no info from git
to determine the release, so it ends up v0.0.0. If that's the case,
we'll now use the release specified in the project() command,
which needs to be updated for each new release.

* cmake: add `--tags` to `git describe`

That way, lightweight tags will also be taken into account, which should
never hurt, but it'll help in cases where, for some mysterious reason or
other, annotated tags don't make it into a clone.

* update releasing.md