meson: use proper dependency fallback for wlroots

meson: use proper dependency fallback for wlroots

Make the subproject wrap file declare the dependencies it provides.

Remove the manual subproject invocation and allow Meson to select
whichever wlroots it finds which satisfies the version requirement --
either a system one via pkg-config, or the subproject fallback.

Use a dependency-generic method of acquiring wlroots configuration info
-- enabled features are present in the pkg-config file and additionally
exported as declare_dependency() variables, so there is no need to do C
preprocessor checks for it.

This ensures that Meson best practices are followed, and also...

Fixes #318