FindGTK2: do not skip target creation if optional dependencies are not found

On some older GTK2 version gio, giomm and cairomm are not available
FindGTK2: Do not use methods not available in older GTK2 version in tests
Merge topic 'FindGTK2-targets' into next

17a53c7 FindGTK2: Do not use methods not available in older GTK2 version in tests
bb024f2 FindGTK2: do not skip target creation if optional dependencies are not found