[Users] Reverted ExternalLibraries/HDF5 to earlier version

I notice the HDF5 didn't build any more on Stampede and Zwicky. The symptoms are errors like these:

/work/00507/eschnett/Cvanilla/arrangements/PITTNullCode/SphericalHarmonicDecomp/src/ExtractMetric.cc(38): catastrophic error: cannot open source file "hdf5.h" #include <hdf5.h> ^

It seems that HDF5's "detect.sh" does not work correctly. Cactus seems to make no attempt to build HDF5.

For the time being, I've reverted the recent changes to thorn HDF5.

-erik