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/cmGeneratorExpressionDAGChecker.h

@@ -0,0 +1,44 @@
+/*============================================================================
+  CMake - Cross Platform Makefile Generator
+  Copyright 2012 Stephen Kelly <steveire@gmail.com>
+
+  Distributed under the OSI-approved BSD License (the "License");
+  see accompanying file Copyright.txt for details.
+
+  This software is distributed WITHOUT ANY WARRANTY; without even the
+  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the License for more information.
+============================================================================*/
+#ifndef cmGeneratorExpressionDAGChecker_h
+#define cmGeneratorExpressionDAGChecker_h
+
+#include "cmStandardIncludes.h"
+
+#include "cmGeneratorExpressionEvaluator.h"
+
+//----------------------------------------------------------------------------
+struct cmGeneratorExpressionDAGChecker
+{
+  cmGeneratorExpressionDAGChecker(const cmListFileBacktrace &backtrace,
+                                  const std::string &target,
+                                  const std::string &property,
+                                  const GeneratorExpressionContent *content,
+                                  cmGeneratorExpressionDAGChecker *parent);
+
+  bool check() const;
+
+  void reportError(cmGeneratorExpressionContext *context,
+                   const std::string &expr);
+private:
+  bool isDAG() const;
+
+private:
+  const cmGeneratorExpressionDAGChecker * const Parent;
+  const std::string Target;
+  const std::string Property;
+  const GeneratorExpressionContent * const Content;
+  const cmListFileBacktrace Backtrace;
+  bool IsDAG;
+};
+
+#endif