While gtkmm installs its html documentation under /usr/share/doc/gtkmm-2.4, glibmm moves it explicitly from /usr/share/doc/glibmm-2.4 to /usr/share/doc/${P}/html. This does bring in problems because it breaks the hyperlink to access the Glib namespace from the index.html page of the installed gtkmm documentation (and all other hyperlinks from the gtkmm documentation). This is *extremely* annoying if you're using devhelp. I'll attach a fixed ebuild for gtkmm-2.9.1 (from the gnome overlay). If you want the same for an older (2.8.x) version, just rename it.
Created attachment 95632 [details] dev-cpp/glibmm/glibmm-2.9.1.ebuild Fixed ebuild.
thanks, in cvs with 2.12.2