don't execute any mpi_info/mpiname in the path

don't execute any mpi_info/mpiname in the path

During MPI setup, the gmxManageMPI.cmake script uses mpi_info/mpiname to
check for and warn about potentially problematic versions. However,
executing these binatries without any path can lead to false positive
warnings e.g. in case if the MPI installation in used does not provide
mpi_info, but another MPI installation's mpi_info is in the path
(which is the case with MacPorts OpenMPI + Xcode's MPI).

To avoid these false positives we don't execute any other
mpi_info/mpiname but the ones found at the same path where the MPI
compiler wrapper resides (if any).

Change-Id: Icd2091dd952f4a140b92045f95a8d0187c1a354f