Attempt to unbreak the build against newer versions of libXxf86dga:

In file included from src/linux/gl_glx.c:57:
/usr/local/include/X11/extensions/xf86dga.h:9:2: warning: "xf86dga.h
is obsolete and may be removed in the future." [-W#$warnings]
/usr/local/include/X11/extensions/xf86dga.h:10:2: warning: "include
<X11/extensions/Xxf86dga.h> instead." [-W#warnings]

While here, add missing USE_XORG components per `stage-qa' note; this
also unbreaks the build when X11 option is not being selected (the bug
previously went unnoticed because it is selected by default).

Reported by: Victor Sudakov