cmGeneratorExpression: Re-write for multi-stage evaluation

The expressions may be parsed and then cached and evaluated multiple
times.  They are evaluated lazily so that literals such as ',' can be
treated as universal parameter separators, and can be processed from
results without appearing literally, and without interfering with the
parsing/evaluation of the entire expression.

Source/cmGeneratorExpressionParser.h

@@ -0,0 +1,45 @@
+/*============================================================================
+  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 cmGeneratorExpressionParser_h
+#define cmGeneratorExpressionParser_h
+
+#include "cmGeneratorExpressionLexer.h"
+
+#include <set>
+#include <vector>
+
+#include "cmListFileCache.h"
+
+class cmMakefile;
+class cmTarget;
+struct cmGeneratorExpressionEvaluator;
+
+//----------------------------------------------------------------------------
+struct cmGeneratorExpressionParser
+{
+  cmGeneratorExpressionParser(
+                      const std::vector<cmGeneratorExpressionToken> &tokens);
+
+  void Parse(std::vector<cmGeneratorExpressionEvaluator*> &result);
+
+private:
+  void ParseContent(std::vector<cmGeneratorExpressionEvaluator*> &);
+  void ParseGeneratorExpression(
+                              std::vector<cmGeneratorExpressionEvaluator*> &);
+
+private:
+  std::vector<cmGeneratorExpressionToken>::const_iterator it;
+  const std::vector<cmGeneratorExpressionToken> Tokens;
+  unsigned int NestingLevel;
+};
+
+#endif