Feature request: add meson option to use system wlroots instead of submodule

[Zoh-j02r]

I was trying to port newm to Void Linux, but it requires the use of wlroots in system, by changing the meson.build to force this behavior for both 0.2 and 0.3 versions it throws errors about missing headers. It would be cool for distribution mantainers to have this functionality.

[jbuchermn]

Hi, I didn't update the readme on pywm, only newm, but both projects are currently unmaintained.

Just a word of warning concerning this change - wlroots doesn't expose the renderer API, so many methods pywm relies on cannot be accessed if you use a precompiled version and pywm relies on compiling wlroots into the resulting lib.