ODK should support parallel builds

We do not explicitly or officially support parallel builds (e.g., `sh run.sh make -j 5 ...`), and it is in fact unclear if our current workflows wouldn’t break in a parallel build.

However, there should be no reason why parallel builds _could not_ be supported. If a workflow fails during a parallel build, that would just indicate that not all dependencies are properly listed – so it would in fact reveal a real issue with the workflow, not just with parallel build.

Not all ontologies may benefit from parallel builds. For large ontologies, the build process is more likely to be limited by memory than by CPU, and for those a parallel build would not help (it could make things worse, in fact).

But for ontologies that are small enough to be CPU-bound rather than memory-bound (or for people who have access to machines with enough memory that their builds are never memory-bound), being able to make parallel builds would be very nice.