cmLocalGenerator: Add ComputeObjectFilenames interface.

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

Source/cmLocalXCodeGenerator.h

@@ -32,6 +32,9 @@ public:
                                 const std::string& rawFlag);
   virtual void Generate();
   virtual void GenerateInstallRules();
+  virtual void ComputeObjectFilenames(
+                        std::map<cmSourceFile const*, std::string>& mapping,
+                        cmGeneratorTarget const* gt = 0);
 private:
 
 };