Multi-core building

Hi all,
This merge request is a re-make yesterday evening's request. It enables multi-core builds.

As long as you applied my other changes from yesterday morning, you'll just need to run setup to generate a build.conf -- no need to delete your conf files again. If you didn't apply yesterday's changes, you'll need to delete all .conf files and re-generate them.

Let me know if you notice any problems. If you do, you can set MAX_BUILD_CORES in build.conf to 1 as a temporary fix until it's resolved.