This means we can't build GTK+ without X11 (eg core-image-weston with x11 removed from DISTRO_FEATURES), as gtk+3 will depend on libepoxy which will depend on libx11 which will refuse to build as x11 isn't in DISTRO_FEATURES.
libepoxy does conditional checks so the libx11 dependency can be condtional on DISTRO_FEATURES. That's no idea but better than nothing.