Fixes for Bluegene/Q

* Moved suffixing code to after where gmxManageBlueGene sets
  GMX_MPI. Since nothing depends on the suffixing until the calls to
  add_subdirectory(), this is safe. There is a complication from the
  way that GMX_MPI means "real MPI" for most of CMake and "real or
  thread MPI" in the source code; that is managed by GMX_THREAD_MPI=ON
  causing GMX_MPI=ON at a certain point. To cope with this, we add the
  "_mpi" suffix in response to GMX_LIB_MPI=on, which is how things
  should have been done back when GMX_MPI was co-opted, except that
  the suffixing was probably being done too early... Altogether, this
  change means that real-MPI binaries are always built with the _mpi
  suffix, like users have come to expect.
* Added compiler suppression for a useless information message that
  is otherwise issued for almost every source file
* Made the C and CXX platform files identical; I'd originally
  thought they could be combined on the command line, but
  that is not true

Change-Id: I26cb782a1b47cf48f47b80ec8c8d0a53db338872