CMake MPI detection updates

[RonRahaman]

In this PR, CMake will test for MPI by using the cache variable CMAKE_CXX_COMPILER, rather than the environment variable CXX.

This will not change the behavior for anyone accustomed to using the env variable. This is because CMake searches for usable compilers in this order and then sets the cache var:

1. Already-set cache variables: (CMAKE_CXX_COMPILER from command line or GUI)
2. Environment variables ($CXX)
3. Common C++ compiler names in $PATH

This should fix a few minor problems. For example, it will fix the behavior of find(HDF5) when re-running CMake.

[RonRahaman]

I also updated this to use FindMPI for CMake versions 3.10+. In 3.10, the FindMPI module was greatly improved and is actually usable :) . See the release notes: https://cmake.org/cmake/help/v3.10/release/3.10.html#modules.

[RonRahaman]

After some more testing, I see that the FindMPI has a few corner cases. So I will put that on the back burner and propose just the first commit.

[paulromano]

Makes sense -- thanks for the fix @RonRahaman!