+ The recipe for cgns seems to be wrong. CMake complains about variables set but
not used (`HDF5_NEEDS_ZLIB`, `HDF5_NEEDS_MPI` and `HDF5_NEEDS_SZIP`) and fails
to find HDF5.
+ This change set removes these variables from the cmake configure line,
replacing them with `'-DHDF5_DIR=%s' % spec['hdf5'].prefix`
+ cgns also has trouble with parallel make, so I set `parallel = False`.
