Config tools (system-config-*) do not work in VNC session.

[Basil Copeland]

I'm getting the following error trying to run any of the
system-config* tools remotely in a VNC session:
-------------
Xlib: connection to "machine:3.0" refused by server
Xlib: No protocol specified

system-config-takeyourpick requires a currently running X server
------------

If it makes a difference, my VNC session is running Xfce4.

Some other Gtk apps give me similar errors trying to run them from the
command line, but will launch from from the Xfce4 task bar.  But not
the system-config-* tools.  Some of the system-config* tools already
are in the Xfce4 Desktop Menu, but they do not launch from a VNC
session (whereas they do from a local session).

The VNC session is for a non-root user, intentionally.  system-config*
tools in the Xfce4 Desktop Menu which require root privileges do
generate a GUI window asking for a prompt, but do nothing further.  I
imagine the same error as described above is simply happening in the
background.

What's going on?