weechat: Patch FindIconv.cmake to make charset.so build

When building weechat with homebrew, the [charset plugin](http://www.weechat.org/files/doc/stable/weechat_user.en.html#charset_plugin) is not built, which makes weechat unable to decode character encodings other than UTF-8.

The charset plugin is not built because cmake thinks libiconv isn't available, which it is (on 10.8.2 at least, it is in `/usr/lib`). This pull request includes a patch for weechat that disables a few cmake checks that fail causing cmake to regard libiconv as unavailable, with the result that libiconv is successfully detected, and charset.so is built.