add_custom_command: Add CODEGEN support

By specifying CODEGEN as an argument to add_custom_command the
custom command will be added to a codegen build target.

The intent is to provide a convenient way for users to get
their generated files without having to build the whole project.

This can be helpful for code analysis tools which can be useful
for IDEs and CI.

Source/cmTarget.h

@@ -173,6 +173,11 @@ public:
   void AddUtility(std::string const& name, bool cross,
                   cmMakefile const* mf = nullptr);
   void AddUtility(BT<std::pair<std::string, bool>> util);
+
+  void AddCodegenDependency(std::string const& name);
+
+  std::set<std::string> const& GetCodegenDeps() const;
+
   //! Get the utilities used by this target
   std::set<BT<std::pair<std::string, bool>>> const& GetUtilities() const;