On one machine only, I'm getting an error when I try to start any image:

--------------
could not find display driver vm-display-X11; either:
  - check that /usr/local/lib/squeak/4.0-2776//vm-display-X11.so exists, or
  - use the '-plugins <path>' option to tell me where it is, or
  - remove DISPLAY from your environment.
---------------

Of course, none of those suggestions worked, and I have been running
Squeak for a long time on this box with no problems.

The same image starts fine on other machines, so I'm guessing
something has gotten hosed up with my Ubuntu 14.04 installation on
this one box?

Any suggestions are appreciated.  Thanks!