A user, Rémi Blanc, reported that Visual Studio Express 2008 defines
DIFFERENCE. For VTK, enum values should have a VTK_ prefix to avoid
potential current and future name clashes. This patch replaces
DIFFERENCE, UNION and INTERSECTION with VTK_DIFFERENCE, VTK_UNION and
VTK_INTERSECTION.

Applications that use the methods SetOperationToDifference,
SetOperationToUnion and SetOperationToIntersection will not need to
update their code.

Applications that use vtkBooleanOperationPolyDataFilter::DIFFERENCE,
vtkBooleanOperationPolyDataFilter::UNION or
vtkBooleanOperationPolyDataFilter::INTERSECTION will need to update their
code.

Change-Id: I26dae4a1c36c359bbe62e070a0d372fc53ccc449