UG contains two implementations of green closures of tetrahedral
elements, and the TET_RULESET macro allows to switch between
the two.  As I recently learned from Peter Bastian, we have been
using the inferior implementation all along, and we should be
enabling TET_RULESET in the future.  As a first step, this patch
allows to set TET_RULESET from the CMake flags.

Incidentally, setting TET_RULESET fixes

  core/dune-grid#27

Beware: when TET_RULESET is set, UG wants to read a table from
the file RefRule.data (in uggrid/lib/ugdata).  This file needs
to be in the current directory, otherwise UGGrid will abort.