Add system's gtk4 if and only if new enough to build our required gtkmm4
If found in system, gtkmm will see it and not build it, so we should pick system's path If not, or too old, gtkmm will build gtk4 and we should not use system's path
If found in system, gtkmm will see it and not build it, so we should pick system's path If not, or too old, gtkmm will build gtk4 and we should not use system's path