Allow HDF5 to be found with CMake >= 3.6.0.

CMake's FindHDF5.cmake underwent some refactoring upstream and it now respects
the HDF5_ROOT variable in more places and separates HDF5_LIBRARIES from
HDF5_HL_LIBRARIES.

Do some slight adjustments to the Alembic port for it to continue working with
the upcoming CMake 3.6.1 (they are no-ops with CMake 3.5.2 which is currently
in the ports tree):

• Set HDF5_ROOT via CMAKE_ARGS so that Alembic does not set it to a nonsensical value.
• Add HDF5_HL_LIBRARIES to ALEMBIC_HDF5_LIBS so that libhdf5_hl.so continues to be passed to the linker.

PR: 210988