I've just been trying to do some digging on possible causes for the issue as I don't really like the libxc11 idea. I salute those involved for getting it to work but I feel in the long run a cleaner solution should be found.

Rebuilding Gtk/Qt was a suggestion I found regarding 1.4 upgrades. I'm not 100% sure if Gtk uses xcb. I'm not sure about time lines here, but have you tried normal names since fmg's vkb work.