Meson has a strict separation of source- and build directory, and
expects anything generated in the latter. That means that in order
to maintain our current setup - shipping the generated CSS in the
repo while also optionally updating it automatically when sassc is
found - we have to fight the build system to some extent, which makes
it less reliable than we would like.
Since we switched to sassc which is a more acceptable build-time
dependency than the original ruby-based tool, just drop the CSS
from the repo and unconditionally generate it from SASS if classic
mode is enabled.
https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/28
GTK+ switched from ruby-based sass to c-based sassc for performance
reasons. It makes sense to follow suit, if not to not require two
tools for the same job ...
https://bugzilla.gnome.org/show_bug.cgi?id=783210
The sass sources now live in a project in GNOME, so they can
be used in multiple projects like gnome-shell-extensions.
Because of that, add gnome-shell-sass as a submodule and import the sass
sources from it.
