FindHDF5: Restore fallback to static libraries

Since commit v3.6.1~6^2~2 (FindHDF5: Fix h5cc arg parsing to work with
homebrew on Mac, 2016-07-11) we treated `HDF5_USE_STATIC_LIBRARIES` OFF
as a requirement for shared libraries.  It is just supposed to be a
preference.  Even if `HDF5_USE_STATIC_LIBRARIES` is not set we should
still fall back to finding static libraries if shared libraries are not
available.

Issue: #16566