nano: enable ncursesw support

Enable support for ncurses widechar by specifying the proper
ncursesw-config when it's enabled, otherwise keep the old trick in place
when it's not to avoid automatically picking up the host/distro one.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>