osx: Fix building gdk-x11

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.

https://github.com/NixOS/nixpkgs/issues/54424
2 jobs for wip/jtojnar/gdk-x11-darwin in 8 minutes and 16 seconds (queued for 5 seconds)
latest
Status Job ID Name Coverage
  Build
passed #196783
build

00:08:16

manual #196784
win32 allowed to fail manual
msys2-mingw32