Check for features.h.

Check for features.h.

        * configure.in, config.h.in:
        Check for features.h.

        * gdk/gdkevents.c (gdk_event_put, gdk_events_queue):
        Fixed bug in linked list append logic by replacing hand-coded
        logic with a call to g_list_append.  Bug found by tml.

        * gdk/gdkimage.c:
        'gcc -ansi -pedantic' on GNU/Linux spews warnings and errors
        in sys/ipc.h, including one specific admonishment to use
        _SVID_SOURCE or _XOPEN_SOURCE.  Manually defined the latter, and
        included features.h if present to obtain the necessary
        side-effects.

        * gtk/gtkclist.c, gtk/gtkeditable.c:
        Eliminates comma-after-final-element warnings.

        * gtk/gtkfontsel.c (gtk_font_selection_init):
        Fixed bug where an intl string was declared as _("x") in
        local var initializer.