Adds CMake build system

Here's an alternate CMake build system for you to consider. It's been working in Psi4 for the last 8 months or so. There's notes, usage, and caveats in README-CMake.md, and a basic CI is working through GitLab-CI. I added the "WIP" label since (1) you'll undoubtably have changes to request and (2) I wasn't expecting the GSL dependency (we've been working from the 3.0.0 tarball) and need to do a bit more testing if that's going to end up in a release (saw a note that it may be removed again).

@susilehtola knows about this in its home at https://github.com/loriab/libxc .