Fix building both X11 and Quartz backends on macOS
This MR allows building GTK with both the X11 and Quartz backends on macOS.
Often, GTK applications work better with the native backend but a lot of apps
only support X11.
To run with X11: GDK_BACKEND=x11 gtk4-demo
- avoid using unsupported AppInfo on macOS
- don't build gtkmountoperation-stub if X11 is enabled
- fix compilation of a C++ test (meson cpp_args)
Edited by Dan Cîrnaț