ncurses: poll() works, but only on Linux

ncurses: poll() works, but only on Linux

ncurses does AC_TRY_RUN() to determine is poll() actually works. As that
doesn't work in cross-compiles, we seed it with 'yes'.

However, MinGW doesn't have a working poll(), so use a :linux override
so that this only applies to Linux builds.

The ncurses build now compiles but doesn't link in MinGW, which is a step
forwards at least.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>