while Ninja is of general benefits to any project build, we don't require it.

Correct logic for building HDF5 with MPI layer

Only find libraries (except MPI) when told to, as too many broken HDF5 system distros out there.

Like MPI, since HDF5 is a big build, introduce logic to make HDF5 build optional.

This will need further minor refinement to handle HDF5-MPI in the future, with our own FindHDF5.cmake from h5fortran etc.