[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 #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The C++ standard requires thread::id objects to be comparable with operator<
and for it to impose a total order.

The current libstdc++ code assumes that a total order can be obtained simply by
comparing the pthread_t objects, which is not portable, but some form of total
order over pthread_t objects is needed.

For w32-pthreads comparing thread sequence numbers should be sufficient.