boomerang: Refactor and fix build.

Thanks to @phreedom for reporting the broken URL used fetchgit, which
was because I deleted my fork repository. Fortunately, in the meantime
other forks got to a more "working" state and being more actively
maintained than my fork. So that's why I switched using @nemerle's fork
now, as it is the the most usable one out there, at least in our case.

One stupid thing I've done in the first place was to use "1.0pre" as the
version and the fork uses "alpha 0.3.2", so it essentially is some kind
of a "downgrade" if you just look at the version.

Fortunately, peer-unreviewed research based on guesswork has shown that
I'm the only one using Boomerang on NixOS, so this shouldn't have a big
impact on the other non-existent users.

Also, this drops dependencies on boehmgc and cppunit, because building
with either one or both will fail at the moment.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>