Use system wlroots

[mldulaney]

Building currently fails on freebsd due to not using system wlroots; I would be interested in working on using the system wlroots install, but I want to know if this would be accepted if I put in the effort.

I also note that being able to do a build without external Internet access (e.g. no git submodules) is required for packaging in many distros.

[sdilts]

Yes! It would be great to be able to use the system install.