Now that these items are backed by tangible files, would it make sense to have a new Makefile rule for building the sed'd files?

Also, to make the Makefile simpler, we can add any needed empty directories to the repository with a small `README.md`, e.g. https://github.com/dropbox/dropbox-js/tree/master/lib

I also think that it would be cleaner to deposit the sed'd files and any other build cruft into a `tmp` directory, as they are not needed by the deployment. Although if this adds significant complexity to the `Makefile`, it might not be worth it.