[CMake] New, improved FindMPI.cmake

[Bill Hoffman]

Alexander Neundorf wrote:

>>
>> There are two known issues:
>>   1) Microsoft's MPI (MS-MPI) has different directories for 32-bit vs.
>> 64-bit libraries. How do we know whether we're building in 64-bit mode
>> to select among the two?
>>   2) FindMPI.cmake has changed a little on the CMake trunk, but I
>> haven't yet incorporated those changes.
> 
> Please put this in the bug tracker so it doesn't get lost.
> 

No need, I am almost done merging it.   Thanks!

-Bill