util-linux: fix ncurses/wchar handling

util-linux: fix ncurses/wchar handling

Since commit 006a328ad6bed214ec3c4d92120510ea37329dd1 ("util-linux: fix
build with ncurses"), we have a build failure that occurs with musl
toolchains when wide-char support is not enabled in ncurses.

The problem comes from the fact that musl provides wchar support (so it
defines wchar_t). But when ncursesw is not available, we currently pass
--disable-widechar which tells util-linux that wchar support is not
available at all (not only in ncurses). When this gets passed to
util-linux configure script, then it defines its own wchar_t, which
conflicts with the musl definition.

So, we should not pass --disable-widechar when BR2_USE_WCHAR=y, which is
what this commit does: it moves the BR2_USE_WCHAR logic outside of the
ncurses logic, and passes --enable-widechar/--disable-widechar as
appropriate.

However, there's a gotcha: util-linux really wants the ncursesw variant
of ncurses when the system has wchar support enabled. We take this into
account by:

 - Enabling ncursesw when BR2_PACKAGE_NCURSES_WCHAR=y. In this case, we
   have ncurses and we have BR2_USE_WCHAR=y, so everything is fine.

 - Otherwise, enabling ncurses when BR2_PACKAGE_NCURSES=y but
   BR2_USE_WCHAR is disabled.

To make sure that the existing util-linux programs that need ncurses
support still build fine, we adjust the Config.in to select
BR2_PACKAGE_NCURSES_WCHAR on BR2_USE_WCHAR=y configurations.

Fixes:

  http://autobuild.buildroot.net/results/19de2a0b12380ddc86dbba0dae3a3877b25f83ff/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>