ENH: Define target and source property LABELS

This creates a new LABELS property for targets and source files.  We
write the labels of each target and its source files in target-specific
locations in the build tree for future use.

Source/cmGlobalGenerator.h

@@ -328,6 +328,9 @@ private:
   std::map<cmStdString, RuleHash> RuleHashes;
   void CheckRuleHashes();
 
+  void WriteTargetLabels();
+  bool WriteTargetLabels(cmTarget* target, std::string& file);
+
   cmExternalMakefileProjectGenerator* ExtraGenerator;
 
   // track files replaced during a Generate