FindMPI: Export `MPI_COMPILER_ID`
It can be helpful to export a MPI_COMPILER_ID
similar to CMAKE_<LANG>_COMPILER_ID
. E.g. MKL
has the option MKL_THREADING
which can depend on the mpi environment
It can be helpful to export a MPI_COMPILER_ID
similar to CMAKE_<LANG>_COMPILER_ID
. E.g. MKL
has the option MKL_THREADING
which can depend on the mpi environment