[Proposal] Use the Meson build system

Created by: ascent12

I've been curious about this build system lately, so I thought I would try to learn it by applying it to wlroots. It seems to be catching on with a few other projects, including libinput and a bunch of GNOME stuff.

I think it's a lot nicer to use than CMake to me, and it seems we can get away with writing a lot less. It also builds a lot faster than CMake.

What I have here is roughly equivalent to what the current CMake implementation does. It builds the library and the examples, but doesn't install/distribute the library yet. Also, it doesn't use subdirectory build files; everything exists in the top-level. I can be written to use subdirectories, but I wrote it this way because it was simpler.

I'm not really pushing this too hard. If you're not interested in changing off of CMake, just close this PR. It was just something I threw together pretty quickly, and I thought it was promising.