[Bug libstdc++/67114] [MinGW64] build failure with POSIX threads enabled

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67114

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Cezary Śliwa from comment #7)
> 1. After applying the patch to GCC 5.2.0, the build fails (I do not see
> PTW32_VERSION defined):

Doh, of course not, because gthr.h isn't included until after os_defines.h,
sorry.