libprotobuf recipe is copied from:
    https://github.com/stephentu/conda-recipes/tree/master/libprotobuf

it is not pushed to conda-recipes since it builds against libc++ instead of libstdc++,
and is replicated here for convenience

the protobuf recipe is pushed mainstream to conda/conda-recipes:
    https://github.com/conda/conda-recipes/tree/master/protobuf