sc-toggle-var message is wrong (half the time)

[Phil Nitschke]

In GNU Emacs 21.1.1 (i386-debian-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2001-10-26 on raven, modified by Debian

In supercite.el, the function sc-toggle-var reports an incorrect message
50% of the time, e.g. calling it 4 times with variable named "foo" gives:
  "foo changed from nil to t"
  "foo changed from t to t"    <-- last value should say "nil"
  "foo changed from nil to t"
  "foo changed from t to t"    <-- last value should say "nil"

This is because the code
  (set-variable variable (not (eval-expression variable)))
always returns "t", not the value of "variable".

So the function does what it is supposed to do, but reports a wrong
message along the way.

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