VS: Prevent generated "rule" files from causing rebuilds

When we generate a ".rule" file to attach a custom command in a VS IDE
project, set the file timestamp to be old enough to prevent the rule
from re-running due to its timestamp.

Source/cmVisualStudio10TargetGenerator.h

@@ -130,6 +130,8 @@ private:
   cmIDEFlagTable const* GetLinkFlagTable() const;
   cmIDEFlagTable const* GetMasmFlagTable() const;
 
+  bool ForceOld(const std::string& source) const;
+
 private:
   typedef cmVisualStudioGeneratorOptions Options;
   typedef std::map<std::string, Options*> OptionsMap;