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

[ralphengels at gmail dot com]

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

--- Comment #13 from ralphengels at gmail dot com <ralphengels at gmail dot 
com> ---
Also needs a small change in pthread.h to guard against including windows.h

#if defined(PTW32_CONFIG_MINGW) && defined(__cplusplus)

change to

#if defined(PTW32_CONFIG_MINGW) && defined(PTW32_BUILD) && defined(__cplusplus)