This allows unit tests to build and run successfully on darwin when
only the Xvfb or XQuartz DDX is built.
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
