[Mingw-w64-public] Strange warnings from x86_64 clang (mingw-w64-x86_64-clang 4.0.0-1) when all warnings are enabled

[Lev Serebryakov]

Hello Mingw-w64-public,

I have project which have (almost) all warnings enabled. It is built well
with clang 4.0 on FreeBSD and mingw-w64-x86_64-gcc 6.3.0-3 on Windows. But
when I try to build it with mingw-w64-x86_64-clang 4.0.0-1 on Windows I got
warnings like this about vsnprintf() and strtod():

===========================
C:/path/to/src.c:1172:9: error: 'vsnprintf' was marked unused but was used [-Werror,-Wused-but-marked-unused]
size = vsnprintf(&c, 1, fmt, argsc);
^
===========================

I have _POSIX_C_SOURCE defined, as I need proper return value from
vsnprintf().

Is it problem of mingw64 or clang? I'm not sure.