The GtkInfoBar is a modern UI component to display messages without
bothering the main window. It'll be used for showing a warning message.

As the GtkInfoBar requires 2.18 (or newer) version of GTK+ library, add
availability check to Makefile too.
Suggested-by: Sunjin Yang <fan4326@gmail.com>
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1338265382-6872-7-git-send-email-namhyung@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>