gtk2 is optional and only used for some example plug-ins. In particular, removing `gtk2` disables building UI module for `$out/lib/lv2/eg-sampler.lv2` (`<http://lv2plug.in/plugins/eg-sampler#ui>`) and the whole `$out/lib/lv2/eg-scope.lv2` (`<http://lv2plug.in/plugins/eg-scope>`).
