ncursesw: improve ncurses compatibility across Linux distributions

Most GNU/Linux distributions will install the ncurses header file
to /usr/include/curses.h.
When NCURSES_WIDECHAR=1, curses.h provides wide character variants
and types.
Thus, we only include curses.h in ncurses/src/mycurses.h.

Debian installs ncursesw (libncursesw5-dev) headers
to /usr/include/ncursesw; to accommodate this, conditionally
add this path to includeDir during build.

For other distributions with non-standard header install
locations, modifying includeDirs in Setup.hs should be sufficient
for compatibility.
ncursesw: add type alias for wint_t

On the C side, `wint_t' is a typedef used by
many of the wide char curses operations.
Previously, the binding assumed that `wint_t' would
be unsigned int (which it is in the current version
of ncurses).
Some users report build failures, however, which appear
to be caused by a violation of the assumption about
the `wint_t' typedef (i.e., they get `CInt' rather than `CUInt').

Perusal of curses.h indicates that `wint_t' should be unsigned
int or not defined at all.
Nevertheless, to work around the issue (and because the typedef could
conceivably change in the future), this patch introduces a
corresponding type alias `Wint_t' on the Haskell side which is set to
the appropriate type by c2hs at compile time.
ncursesw (mycurses.h): remove redundant include wchar.h

curses.h includes wchar.h