OS X: Add CMP0042 to enable MACOSX_RPATH by default

Also adding documentation for CMAKE_MACOSX_RPATH, and improving
documentation for MACOSX_RPATH.

Source/cmLocalXCodeGenerator.h

@@ -29,6 +29,8 @@ public:
   virtual ~cmLocalXCodeGenerator();
   virtual std::string GetTargetDirectory(cmTarget const& target) const;
   virtual void AppendFlagEscape(std::string& flags, const char* rawFlag);
+  virtual void Generate();
+  virtual void GenerateInstallRules();
 private:
 
 };