Creates a new build engine action.

        Creates on bjam side an action named 'action_name', with
        'command' as the command to be executed, 'bound_variables'
        naming the list of variables bound when the command is executed
        and specified flag.
        If 'function' is not None, it should be a callable taking three
        parameters:
            - targets
            - sources
            - instance of the property_set class
        This function will be called by set_update_action, and can
        set additional target variables.

def register_action (self, action_name, command='', bound_list = [], flags = [],
                         function = None):
        """Creates a new build engine action.

        Creates on bjam side an action named 'action_name', with
        'command' as the command to be executed, 'bound_variables'
        naming the list of variables bound when the command is executed
        and specified flag.
        If 'function' is not None, it should be a callable taking three
        parameters:
            - targets
            - sources
            - instance of the property_set class
        This function will be called by set_update_action, and can
        set additional target variables.
        """
        assert isinstance(action_name, basestring)
        assert isinstance(command, basestring)
        assert is_iterable(bound_list)
        assert is_iterable(flags)
        assert function is None or callable(function)

        bjam_flags = reduce(operator.or_,
                            (action_modifiers[flag] for flag in flags), 0)

        # We allow command to be empty so that we can define 'action' as pure
        # python function that would do some conditional logic and then relay
        # to other actions.
        assert command or function
        if command:
            bjam_interface.define_action(action_name, command, bound_list, bjam_flags)

        self.actions[action_name] = BjamAction(
            action_name, function, has_command=bool(command))