Merge ROLLNETWORK make var into ROLLMPI.  For consistency, allow ROLLMPI to
include a version similar to ROLLCOMPILER.
Change default MPI.
Merge pull request #2 in APS/fftw-roll from feature/remove-rollnetwork-make-var to master

* commit 'd0fed201df82e25d45f138c6c57bd4b60f419106':
  Change default MPI.
  Merge ROLLNETWORK make var into ROLLMPI.  For consistency, allow ROLLMPI to include a version similar to ROLLCOMPILER.