[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 #19 from ralphengels at gmail dot com <ralphengels at gmail dot com> ---
Had problems with the define not being pulled in from os_defines.h (i think a
better place would have been gthr_posix.h since that gets pulled in at build
time) hence the change, also if using another posix thread implementation it
would break using that instead since your define would then be undefined
leading to a build error. Besides that better implementations are welcome :)
this was just an example of a possible fix.