Avoid a few compilation warnings in Objective-C code.

Avoid a few compilation warnings in Objective-C code.

* src/nsfns.m (Fns_frame_restack): Remove unused variable 'flag'.

* src/nsmenu.m (ns_update_menubar): Remove unused variable 'pool'.

* src/nsterm.m (focus_view, hide_bell): Define conditionally.
(ns_update_end): Define variable 'view' conditionally.
(ns_redraw_scroll_bars): Don't define unused function.
(copyRect): Don't perform arithmetic on 'void' pointers.
(nswindow_orderedIndex_sort): Make static.