from gs-plugin-appstream.c:25: /usr/include/libappstream-glib/as-app.h:326:8: note: declared here void as_app_add_category (AsApp *app, ^ make[4]: *** [libgs_plugin_appstream_la-gs-plugin-appstream.lo] Error 1
Works with dev-libs/appstream-glib-0.4.0 (and everything else seems to work too) AFAIK appstream-glib-0.5.x is for gnome-3.17, maybe it's been keyworded a bit too soon.
I have rebuilt all reverse deps and all but this one look to compile and work ok, also, as far as I saw when bumped it, appstream-glib doesn't follow the release cycles of odd versions being "devel" and pairs being "stable"
I think you're right about the versioning, but in gnome-software's configure.ac, in 3.16.5 it's checking for >=appstream-glib-0.4, and it's in 3.17 that it's checking for >=appstream-glib-0.5 (and it appears to need >=appstream-glib-0.5.1 in coming 3.18)
Moved to tree a version compatible with recent appstream-glib.