Optionally build MPI

[drwells]

some machines (macs, for instance; also longleaf) don't have MPI - that's really the last dependency that we need. I think we should make it optional and off by default.

One way to get MPI working easily would be to download it with PETSc and then set CC et al to the correct values afterwards. We could also just build it from source, though that requires a touch of care (openMPI does not support out-of-source builds, which is what we presently do for everything). @brynbarker would you like to give this a try? We can chat about it tomorrow if you are interested.