Add Target API to determine if an include is a system include.

The implementation can be modified later so that system includes
can be determined on a per-target basis.

Source/cmGeneratorTarget.h

@@ -72,6 +72,8 @@ public:
   /** Get the include directories for this target.  */
   std::vector<std::string> GetIncludeDirectories(const char *config);
 
+  bool IsSystemIncludeDirectory(const char *dir, const char *config);
+
 private:
   void ClassifySources();
   void LookupObjectLibraries();