How to exclude a major mode from a hook

[Cecil Westerhof]

I had the following in my .emacs:
    (add-hook 'before-save-hook 'delete-trailing-whitespace)

But I also use Gnus and a signature starts with '-- ' and now the
space is deleted and it is not a signature anymore.

So I rewrote it to:
    (add-hook 'before-save-hook (lambda () (when (not (string= major-mode 
"message-mode"))
                                             'delete-trailing-whitespace)))

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof