FindHDF5: Fix search with HDF5_ROOT

Refactoring in commit v3.6.0-rc1~72^2 (HDF5: Rework component searching
to correctly find HL for all bindings, 2016-05-12) turned off a large
amount of its logic when HDF5_ROOT is set.  This caused use of the hdf5
compiler wrapper to extract all needed libraries (e.g. z, dl as
dependencies of hdf5 static libraries) to be skipped when using
HDF5_ROOT.

Fix the search logic to honor HDF5_ROOT in all code paths.  Restrict the
search for hdf5-specific components to this root, but allow external
libraries to be found anywhere.

Fixes: #16566