[CMake] FindMPI.cmake updated in CVS trunk

[Doug Gregor]

I've updated the FindMPI module on the CVS trunk with proper support
for Microsoft's MPI (MS-MPI), both 32- and 64-bit. It's revision 1.18
in the repository.

If there's another RC for 2.6.0, I suggest merging this in. But this
is a minor tweak, so it's not worth rolling another RC just for this
change.

  - Doug