Port commit from gitorious copy of sofia-sip our code now differs but this issue would still be a concern for OS X

commit ee51fa4e2993ab71339e29691aec8b924c810c53
Author: Frode Isaksen <frode.isaksen@bewan.com>
Date:   Thu Aug 18 16:40:58 2011 +0300

    su: fix su_time() on 64-bit OS X

        The field tv_sec in struct timeval is 64bits instead of 32bits as in
	    su_time_t, so you cannot cast su_time_t to struct timeval.