configure: Define NCURSES_WIDECHAR if we're using curses

We want the wide character functions from the ncurses header.
Unfortunately it doesn't provide them by default, but only
if either:
 * NCURSES_WIDECHAR is defined (for ncurses 20111030 and up)
 * _XOPEN_SOURCE/_XOPEN_SOURCE_EXTENDED are suitably defined

So far we have been implicitly relying on the latter, because
for GNU libc when we define _GNU_SOURCE this causes libc
to define the _XOPEN_SOURCE macros for us. Unfortunately
this doesn't work on all libcs, because some (like OSX and
musl libc) do not define _XOPEN_SOURCE when _GNU_SOURCE
is defined.

We can't fix this by defining _XOPEN_SOURCE ourselves, because
that also means "and don't provide any functions that aren't in
that standard", and not all libcs provide any way to override
that to also get the non-standard functions. In particular
FreeBSD has no such mechanism, and OSX's _DARWIN_C_SOURCE
doesn't reenable everything (for instance getpagesize()
is still not prototyped if _DARWIN_C_SOURCE and _XOPEN_SOURCE
are both defined).

So we have to define NCURSES_WIDECHAR. (This will only work
if your ncurses is at least 20111030, as older versions
don't honour this macro.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1496414138-7622-1-git-send-email-peter.maydell@linaro.org
