Undoing last patch because:

- just giving a more verbose error message doesn't solve the problem

- The issue has been solved within UG.  The UG build system now aborts
  when --enable-dune is set but not __cplusplus.  I had just forgotten
  to close the issue.  Beg your pardon.

Please try to avoid unrelated indentation and whitespace changes in
patches, as the diffs become more difficult to read.



[[Imported from SVN: r4805]]