Instead of requiring sassc to be installed add meson subprojects
which build libsass and sassc (currently both forks of mine, tested
under linux/mingw/msvc) when needed.
This allows us to drop the generated .css files and build scripts from git.
See #1502
We rely on a specific minimum version of gtk-doc to be able to build the
GTK API reference for the new API. In order to be able to use gtk-doc as
a subproject, though, we need to use a recent version of Meson.