A new option WITH_GTK3 has been added to the cmake configure system to enable compilation under Gtk version 3.The flag HAVE_GTK3 is also introduced to select the new Gtk3 code in the source files. (Gtk2 is disbled when Gtk3 is selected).
window_gtk.cpp has been modified to remove obsolete (and deleted from libgtk 3) and introduce new Gtk3 code in its place when compiled for Gtk3.
To compile for Gtk2, disable WITH_GTK3 in cmake. To build for Gtk3 both WITH_GTK and WITH_GTK3 must be selected.
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