Homebrewでautoconfを入れようとしてるんですけど
Warning: Could not link autoconf. Unlinking...
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
You can try again using `brew link autoconf'

と出てしまって brew link autoconfをすると

Warning: Could not link autoconf. Unlinking...
Error: Could not symlink file: /usr/local/Cellar/autoconf/2.69/share/emacs/site-lisp/autotest-mode.elc
/usr/local/share/emacs/site-lisp is not writable. You should change its permissions.

と出てしまうんですけど何が原因なんでしょうか？
PATHも一番に/usr/local/binがきてます。
OSはMountain Lionです。