The glibmm-2.6.1 package requires at least glib-2.6. However, the ebuild only specifies ">=dev-libs/glib-2.4" Reproducible: Always Steps to Reproduce: 1) emerge glibmm Expected Results: It should have added glib-2.6 to the dependency list. Emerging glib manually to at least 2.6 fixes the problem.
Fixed.