Fix compilation issues for GNUStep and GLIB conflicts.

Fix compilation issues for GNUStep and GLIB conflicts.

* configure.ac (HAVE_GLIB): Only set XGSELOBJ if HAVE_NS = no.
(with_file_notification): Don't set to gfile if with_ns = yes.

* src/process.c (wait_reading_process_output): Check for NS before GLIB.
GLIB may be linked in due to rsvg, but ns_select must be called.

* src/xgselect.c (xg_select): Remove call to window_system_available
and g_main_context_pending at the top, so Gdk events (i.e. file
notify) are processed when Emacs is started with -nw.

* src/xgselect.c: Remove unneeded include xterm.h