Fix build issues on beauregard.
Fix a build system heuristic.

PISM should not use MPI_LIBRARY, because it may contain the name of
the MPI C++ library from a different installation.
SIA mass conservation test prints the "pismv" command it runs.
Rename IceModelVec::stencil_width() back to get_stencil_width()

(This is more consistent.)
Minor cleanup.