Make INTERFACE determined properties readable in generator expressions.

The properties are evaluated as link-dependent interface properties when
evaluating the generator expressions.

Source/cmGeneratorExpressionDAGChecker.h

@@ -35,6 +35,9 @@ struct cmGeneratorExpressionDAGChecker
 
   void reportError(cmGeneratorExpressionContext *context,
                    const std::string &expr);
+
+  bool EvaluatingLinkLibraries();
+
 private:
   Result checkGraph() const;