ncursesw: add type alias for wint_t

wint_t is a typedef and could conceivably change ...
also, it appears to only be defined if NCURSES_WIDECHAR is set.
ncurses: improve Setup.hs

Most installations should have /usr/include/curses.h.
For debian support, simply append /usr/include/ncursesw to includeDirs
(Debian users must take care to have libncurses5-dev installed).

SunOS and CygWIN probably need some extra care ...