https://bugzilla.redhat.com/show_bug.cgi?id=1170875
--- Comment #11 from Zbigniew Jędrzejewski-Szmek zbyszek@in.waw.pl --- Ah, OK, that's why I couldn't find it. According to [1] the submodule does not have a stable api and is supposed to be "linked" wherever it is needed. So including it in budgie-desktop is allowed by the packaging guidelines, as a "copy library".
[1] https://mail.gnome.org/archives/gnomecc-list/2012-October/msg00003.html
Neverthless, all four projects use the same git version of the submodule. It seems a bit unfortunate to have it compiled separately four times. It would be great to de-duplicate those copies at some point, but it's nothing to hold up the review over.