hdf5: avoid empty "enable parallel" flag
This flag ends up in hdf5-config
and, if empty, ends up becoming
set(HDF5_ENABLE_PARALLEL)
which is unset
and then confuses VTK's
FindHDF5
which forwards it to HDF5_IS_PARALLEL
and trips the NOT DEFINED HDF5_IS_PARALLEL
check inside of VTK itself.