Return the major version number for Gtk+ that was linked.
Note that this is not necessarily the same as for GtkAda. It could also
be different when your application is running, if the dynamic linker
find some other GtkAda library. Use Gtk.Main.Check_Version to ensure
that the two versions are compatible
If the version is 1.2.6, returns 1.
Binding from C File version 2.8.17