Fix issues with using system HDF5 with MPI support.

When building VTK (or ParaView) system HDF5, and if that HDF5 was built with MPI
support enabled, VTK would fail to build. The solution was to always manually
set the HDF5 include dirs and libraries to include the MPI dirs and libraries.
This is needed since hdf5 includes mpi.h in one of its API headers. This fix
addresses that issue by detecting if system HDF5 is built with MPI support (i.e.
HDF5_IS_PARALLEL is set to true) and if so, doing a find_package(MPI) to update
the HDF5 variables to include the MPI ones as well.

This addresses BUG #14363 and should make it easier to build ParaView/VTK on
various supercomputers where this (i.e. using system HDF5 with MPI support) is
commonly the case.

Change-Id: I59c210595496b07c0199bc969058fdc6f38fa813
Merge topic '14364_fix_hdf5_mpi' into master

73414b02 Fix issues with using system HDF5 with MPI support.