emerge gtkmm without stating a version-number uses 1.3.15 on my system which is unstable and does not build /usr/bin/gtkmm-config which is a prerequisite to fwbuilder and others. So I had to use "emerge gtkmm-1.2.9" instead. Hope this is not the normal with emerge.
ugh, i should have known this would pop up soon, gtkmm + etc should be remasked gtkmm 1.3.x will build pkg-config .pc files instead of gtkmm-config, (the -config programs are deprecated in favour of pkg-config)