curses: Use cursesw instead of curses

Use ncursesw package instead of curses on non-mingw, and check a few
functions.
Also take cflags from pkg-config, since cursesw headers may be in a
separate, non-default directory.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-id: 20161015195308.20473-3-samuel.thibault@ens-lyon.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>