Todd Gamblin authored Dec 29, 2010 and Brad King committed Mar 15, 2011

Adds support for:

- MPI_<lang>_COMPILER and other useful variables for C, CXX, Fortran
- Better compiler interrogation (handles mvapich)
- Supports specifying an MPI compiler name directly on the command line
  without and absolute path, e.g.: cmake -D MPI_CXX_COMPILER=mpixlC
- Better compiler name searching tries to match MPI compiler to regular
  CMAKE_<lang>_COMPILER_ID, if it's available.

Gets rid of:

- MPI_LIBRARY, MPI_EXTRA_LIBRARY cache variables.  These and other old
  vars are still exported for backward compatibility, but they're not
  cached.