cmGlobalGenerator: Create a non-virtual 'DoGenerate' method

Make the virtual 'Generate' method protected.  Make 'DoGenerate'
the main entry point to generation.  This gives cmGlobalGenerator
a chance to do some early operations before the individual
generator-specific implementations take over.

Source/cmGlobalXCodeGenerator.h

@@ -64,13 +64,6 @@ public:
     std::vector<std::string> const& makeOptions = std::vector<std::string>()
     );
 
-  /**
-   * Generate the all required files for building this project/tree. This
-   * basically creates a series of LocalGenerators for each directory and
-   * requests that they Generate.
-   */
-  virtual void Generate();
-
   /** Append the subdirectory for the given configuration.  */
   virtual void AppendDirectoryForConfig(const std::string& prefix,
                                         const std::string& config,
@@ -91,6 +84,8 @@ public:
 
   virtual bool SetGeneratorToolset(std::string const& ts, cmMakefile* mf);
   void AppendFlag(std::string& flags, std::string const& flag);
+protected:
+  virtual void Generate();
 private:
   cmXCodeObject* CreateOrGetPBXGroup(cmTarget& cmtarget,
                                      cmSourceGroup* sg);