BullXMPI / FindMPI fix

- at least for mpiInfo, Bullxmpi tries to link against the
  deprecated MPI C++ Interfaces (whyyyyy?)
- since it is hard to find what depencency resolves the same
  problem in PIConGPU, we add the same work-around here
- another option: it would be possible to use the FindMPI
  deprecated var MPI_LIBRARIES instead, but we avoid that