Multi-core make

(Please be sure that no one merges with this request until we're certain that it doesn't break anything for anyone.)

This merge enables multi-core building. By default it will use all available cores. If desired, you can specify the number of cores to use in build.conf.

Note: After these changes are merged you'll have to run setup.conf so it can generate build.conf. You do *not* need to delete your existing conf files, though -- we haven't been generating build.conf in the past as it was empty.

It would be great to get some feedback on whether these changes work for everyone, particularly people who have had problems with multi-core building in the past. If you have any feedback, please post it in issue #32.

Perry