This is a near-total rewrite of FindMPI implementing the following
features:

Environmental improvements:

- C++ settings can now be picked from C settings if no dedicated C++
  settings are found (some installs of IBM Platform MPI)
- Fortran settings can now be found on MPICH-1, MVAPICH-1, Microsoft
  MPI and MPICH-2 on Windows.

- Improved flag procession logic: Flags related to security enhancements
  of MPI itself, and unrelated to MPI usage (-fstack-protector-strong on
  e.g. RHEL 7 for MPICH and MVAPICH) will be filtered out properly now.
  -pthread is now being handled. -fno-strict-aliasing, which may be needed
  while compiling MVAPICH (e.g. RHEL 7's MVAPICH package) is also
  filtered.

Features added:

- MPI version can be determined for each language and is exposed as the
  package version
- MPI functionality is being tested to ensure the correctness of
  settings
- MPI-2 C++ bindings can be suppressed if desired by the project or
  user. For older MPIs, th...