Use swaret to install it. If you dont have swaret, go to swaret.sf.net, download and install the package. then open /etc/swaret.conf.new, change VERSION= to current, make sure:
appears with the other REPOS_ROOTs, then save it as swaret.conf
Then (as root) run swaret --update and swaret --install gtkmm (and swaret --upgrade -a if you want to update all packages) Then run swaret --dep.