cairomm is too new in modulesets-stable
The version of cairomm in modulesets-stable/gtk-osx-gtkmm.modules is 1.15.5, and that seems to be too new to be compatible with the other packages when building gtkmm3. For example, cairomm 1.15.5 requires sigc++-3.0, but all other packages in modulesets-stable use sigc++-2.0.
I changed the cairomm version to 1.13.1, and then the build of gtkmm3 passes.