kconfig: get rid of stray a.o, support ncursesw

scripts/kconfig/lxdialog/check-lxdialog.sh uses gcc to check for
what libraries are present. Redirect output to /dev/null
so we do not generate an a.out.
Also included support for ncursesw - so if present prefer that
instead of ncurses.
The order is now (first is preferred):
1) ncursesw
2) ncurses
3) curses

The latter is to support SunOS.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>