Add support for parallel/concurrent builds

Apologies if this is a bad request, I have read the docs and skimmed the code and I neither see support for this feature nor any statement that such a feature would not be supported. I think the request is self-explanatory. As a naive implementation, this could be done like: ```bash make -j build/{borg,compat,<all clones here>} # Add to Makefile: build/<all other clones>: build/borg build/compat ```