Always define __USE_MINGW_ANSI_STDIO as 0 or 1 in _mingw.h

We check state of __USE_MINGW_ANSI_STDIO in many places.
This patch checks all corner cases for __USE_MINGW_ANSI_STDIO
in one place in _mingw.h and then we have easy and consistent check
-- it is 0 or 1.

Signed-off-by: Mateusz Brzostek <mateuszb@poczta.onet.pl>
Signed-off-by: Liu Hao <lh_mouse@126.com>