Log Message:
Update gtkmm to 2.24.5

2.24.5 (stable)

This release can be used together with glibmm up to at least 2.49.5.
It's recommended that you use gtkmm3 instead of gtkmm2. The support for
gtkmm2 is minimal, and diminishing.

Build:
* Update the MSVC projects (Chun-wei Fan)
* Use C++11 if the compiler supports it (Murray Cumming)
* Update to be compatible with new versions of glibmm
  (Kjell Ahlstedt, Murray Cumming)

Documentation:
* Improve the documentation of Gtk::ComboBoxText
  (Kjell Ahlstedt) Bug #766039 (Omega Phil)