slang: add ncurses optional dependency

Ncurses check need to be disabled if ncurses package is not
selected.
If we don't do that, host's ncurses5-config may be used by
configure script.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>