gtkmm: Fix building on Visual Studio
gtkmm: Fix building on Visual Studio

When we invoke gendef.exe in this case, we must include both the static gdkmm
library and the (temporary) static gtkmm library, so that we can have the
complete list of symbols that we want to export from the final gtkmm DLL.

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm