gpi_make still trying to import site-package packages as node libraries

During a build, if the `--ignore-system-libs` flag isn't included, `gpi_make` is still looking two directories upstream and finding lots of python packages in `site-packages`. These are then included as `include_dirs` for the build. I don't know that this is a problem (the build still works fine), but the messages displayed are a little confusing.

The library search setup feels a bit hacky in general, so I think this could be part of a clean-up there. Can we come up with a good list of behaviors we want out of `gpi_make`?