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.