Fix GNUstep compilation warnings

Fix GNUstep compilation warnings

* src/Makefile.in (NON_OBJC_CFLAGS): Add `-Wnested-externs'.
* src/emacs.c (load_pdump): Only define exec_bufsize if
!NS_SELF_CONTAINED.
* src/image.c (ARGB_TO_ULONG): Don't define if HAVE_NS.
* src/nsterm.m (ns_destroy_window): Pacify incorrect GCC
warning.
(ns_query_color): Make static.
(ns_run_loop_break): Fix old-style definition.
([EmacsView toggleFullScreen:]): Remove unused variable.