Patch for tmux on Snow Leopard (#18776)

This patch addresses issue #18776 by applying the patch used by MacPorts.  This was submitted to the tmux mailing list (http://sourceforge.net/mailarchive/forum.php?thread_name=CAJfQvvc2QDU%3DtXWb-sc-NK0J8cgnDRMDod6CNKO1uYqu%3DY5CXg%40mail.gmail.com&forum_name=tmux-users) so hopefully it will be incorporated there and we can remove the patch then.