Factor out duplicate VS target dependency code

Compute VS target dependencies in cmGlobalVisualStudioGenerator when the
main global dependency analysis is done.  Use these results in each of
the VS generators instead of duplicating the analysis.

Source/cmGlobalVisualStudioGenerator.h

@@ -93,6 +93,11 @@ protected:
   virtual void GetTargetSets(TargetDependSet& projectTargets,
                              TargetDependSet& originalTargets,
                              cmLocalGenerator* root, GeneratorVector const&);
+  virtual bool ComputeTargetDepends();
+  class VSDependSet: public std::set<cmStdString> {};
+  class VSDependMap: public std::map<cmTarget*, VSDependSet> {};
+  VSDependMap VSTargetDepends;
+  void ComputeVSTargetDepends(cmTarget&);
 
   bool CheckTargetLinks(cmTarget& target, const char* name);
 private: