I've wondered why gtkmm-2.8.1's pkg-config only works when gtkmm-2.4 is passed as an argument and I've found out why: In the ebuild itself (in dev-cpp/gtkmm/gtkmm-2.8.1.ebuild), it uses a SLOT="2.4". Why?
No, it doesn't. Same for 2.6
But gtkmm-2.4.x is already "slotted", doesn't make sense to slot 2.8.x in there as well? Is there a reason for it?
has to do with the soname, try doing: equery f =gtkmm-2.8.1 | grep libgtkmm-2.4.so and you can see the soname is still 2.4.