cmLocalGenerator: Add ComputeObjectFilenames interface.

Implement it in the local generators and use it in the global
generators.

Source/cmLocalVisualStudioGenerator.h

@@ -61,6 +61,10 @@ public:
 
   virtual void AddCMakeListsRules() = 0;
 
+  virtual void ComputeObjectFilenames(
+                        std::map<cmSourceFile const*, std::string>& mapping,
+                        cmGeneratorTarget const* = 0);
+
 protected:
   virtual const char* ReportErrorLabel() const;
   virtual bool CustomCommandUseLocal() const { return false; }