Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229
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