configure.ac: improve X11 detection
configure.ac: improve X11 detection

Use gtk-x11-[23].0.pc to detect whether we want an x11 build or
not. Don't AC_SUBST X11_CFLAGS and X11_LIBS since they are unused.

freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,dezelin/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,mathslinux/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,dezelin/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,dezelin/spice-gtk,dezelin/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,mathslinux/spice-gtk