send-mail-function initialized in ldefs-boot (using window-system)

[David Reitter]

The default value for send-mail-function is supposed to be different  
depending on the system. This doesn't work, at least not on Darwin,  
where it is supposed
Currently, we have (lisp/mail/sendmail.el):

;;;###autoload
(defcustom send-mail-function
  (if (and window-system (memq system-type '(darwin windows-nt)))
      'mailclient-send-it
    'sendmail-send-it)

This is included in ldefs-boot.el / loaddefs.el and is loaded before  
dumping.
At that time, window-system is nil, of course, and the default is set  
prematurely.
I'm not sure what the right solution for this is.

I've seen another use of window-system in loaddefs.el - in the  
definition of tex-dvi-view-command. There, the expression is quoted,  
but this value is evaluated before use in that case.