Make: Support shallow git clones for testing

Make: Support shallow git clones for testing

Since commit 1ec9b94, we derive the version number from git tags with
build-aux/git-version-gen.  When a shallow clone doesn't include a
suitable tag, this fails, and make refuses to build anything.  Since
Travis uses git-clone --depth=50, it'll break as soon as we've got
more than 50 commits since the last release.

Support arbitrarily shallow clones for limited purposes like testing
by falling back from a proper V.N-H version number to UNKNOWN-H.

To guard against use of such builds for other purposes, log a warning
on server startup, and print one on player login.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>