Enable graphical benchmark by default in cmake

Toggle between point to point and generic 6dof constraint for mouse picking in the demos

Use a 'equal vertex thresdhold' in the btVoronoiSimplexSolver of  0.0001f by default.
This can be disabled (or configured) using defaultCollisionConfiguration->getSimplexSolver()->setEqualVertexThreshold(0.f);
See http://code.google.com/p/bullet/issues/detail?id=305

src/BulletCollision/NarrowPhaseCollision/btVoronoiSimplexSolver.cpp

@@ -289,7 +289,11 @@ bool btVoronoiSimplexSolver::inSimplex(const btVector3& w)
 	//w is in the current (reduced) simplex
 	for (i=0;i<numverts;i++)
 	{
+#ifdef BT_USE_EQUAL_VERTEX_THRESHOLD
+		if ( m_simplexVectorW[i].distance2(w) <= m_equalVertexThreshold)
+#else
 		if (m_simplexVectorW[i] == w)
+#endif
 			found = true;
 	}