build-sys: simplify checking for x11
build-sys: simplify checking for x11

We no longer need the gtk+-quartz/win32 check, however we can simplify
the x11 check if building gtk+-x11

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>
Acked-by: Victor Toso <39b7599ff73bbf5db531748ba5250f21fa7120ff@redhat.com>

flexVDI/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk