How to add to lists defined with c-lang-defconst?

Question

I'm trying to add a C++ mode hook to add C++11 keywords to the appropriate keyword lists (which are defined in lisp/progmodes/cc-langs.el in the Emacs source code). As a minimal example, I started with c-modifier-kwds and tried adding the following to my .emacs file:

(add-hook 'c++-mode-hook
          (lambda ()
            (c-lang-defconst c-modifier-kwds
                             c++
                             (append '("thread_local" "noexcept")
                                     (c-lang-const c-modifier-kwds))))
          t)

(I copied the c-lang-defconst statement from this C++11 mode implementation. However, I don't want to create a whole new mode, I just want to add to the regular C++ mode.)

That didn't work (I didn't really expect it to, either). I also tried the following:

(add-hook 'c++-mode-hook
          (lambda ()
            (setq (c-lang-defconst c-modifier-kwds)
                  (append '("thread_local" "noexcept")
                          (c-lang-const c-modifier-kwds))))
          t)

That didn't work either.

What's the right way to do this?

If there is no right way, what's a good hackish way to do this?