From Peter Wu: on systems other than Windows, skip the GUI tests if the

From Peter Wu: on systems other than Windows, skip the GUI tests if the
DISPLAY environment variable isn't set.

From me: note that this won't be the right thing to do on OS X if we
switch to a toolkit that doesn't use X11, and that this may or may not
be the right thing to do with toolkits using Wayland or Mir directly.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@52182 f5534014-38df-0310-8fa8-9805f1628bb7