[petsc-dev] Configure argument check

I believe Jed has suggested before that we multiplex this check over different architectures. This is also starting to bite me and I looked at MPI.py. I think just by changing args.petsc to args.<arch>.petsc we can achieve this. Of course, it will not work for the unfortunate poop that use make.inc, but we can clean that up later. Any objections to this change? Matt -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener