treat pylibxc as independent project

This allows pylibxc to be built separately and after the main library. It's handy for packaging when you want to build the lib once and then do trivial builds for each Python. Soon to be used in the conda-forge packages.

This isn't in polished form (in particular, I've put relatives of this keyword in three packages with slightly different names and usages in the past few weeks), so let me know when the project is ready for cmake changes, and I'll finalize the proposal.