Update meson to use features instead of combos
Created by: ascent12
Fixes #1082 (closed)
Uses the new Meson option type to handle optional features better. This bumps the required meson version quite high, but it's currently in Arch Linux stable, but most other distros were behind the required Meson version anyway. Not to mention that it's very easy to run Meson straight from git.
I also changed how the difference between libsystemd/libelogind was handled. because I think it's cleaner this way.
This breaks the "build system API", so anyone running any script building this (e.g. AUR or https://github.com/swaywm/wlroots-rs) will need to update.