I just re-built using this commit ( http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-25&id=711ca362e7f8ca7c4f664dc2fe60bc5fa4e4f4fe ).

Now the earlier error went away, but I get 

  Symbol’s value as variable is void: prev

I guess the read call should be inside the let:

(let ((str "TEST")

      (pos 0)

      (prev nil))

  (read (lambda (&optional ch)

          (cond

           (ch (push ch prev))

           (prev (pop prev))

           ((< pos (length str))

            (prog1 (aref str pos)

              (setq pos (1+ pos))))))))

Above returns "TEST".