CMake ability to find HDF5 is fragile

A user offered a patch in Nov 2017 to address a problem they were having in forcing MOAB to find a custom install of HDF5. That patch became PR #375.

I believe those changes may have been suitable for that user, but are not the best solution overall. In particular, following this change, it may not be possible to build from CMake against a standard installation of HDF5.