osx: Fix building gdk-x11
This is forward port of !536 (closed)
Glib linked against Apple libraries does not provide <gio/gdesktopappinfo.h> header. When building X11 GTK backend on MacOS against such Glib, it fails because it cannot find the header. X11 backend is still required by programs like vim or Emacs.
Let’s use the same trick gtk-launch.c uses to solve this issue.