pangomm has !<dev-cpp/gtkmm-2.13 in its depends, but it doesn't conflict with gtkmm-2.2.1, (likely not with 1.2.9 either, but i don't have that installed.)
Thanks for the report, assigning to maintainers
I put in EAPI=1 slotted deps for the blocker. That should do the trick. Fixed in portage without a revbump. Thanks