Make build_gtk4=false and build_legacy=false a build error
If you try to build gnome-desktop without the new libraries (build_gtk4=false) and without the legacy ones (build_legacy=false) then there's nothing to build, so we should error out.
If you try to build gnome-desktop without the new libraries (build_gtk4=false) and without the legacy ones (build_legacy=false) then there's nothing to build, so we should error out.