Add API to check that dependent target properties form a DAG.

Initially this will only be used to check for self-references, but
can be extended to check for cycles when chaining properties of other
targets.

Source/cmGeneratorExpression.h

@@ -25,6 +25,7 @@ class cmMakefile;
 class cmListFileBacktrace;
 
 struct cmGeneratorExpressionEvaluator;
+struct cmGeneratorExpressionDAGChecker;
 
 class cmCompiledGeneratorExpression;
 
@@ -60,7 +61,8 @@ class cmCompiledGeneratorExpression
 public:
   const char* Evaluate(cmMakefile* mf, const char* config,
                        bool quiet = false,
-                       cmGeneratorTarget *target = 0) const;
+                       cmGeneratorTarget *target = 0,
+                       cmGeneratorExpressionDAGChecker *dagChecker = 0) const;
 
   /** Get set of targets found during evaluations.  */
   std::set<cmTarget*> const& GetTargets() const