[cpp-threads] Fixing the thread type

[Peter Dimov]

Alexander Terekhov wrote:

> I can comment that apart from emulating pthreads on top of
> Win32-proper a la pthreads-win32 (better critical sections, rwlocks
> without read-or- write preference, sema based condvars, and DCSI
> were mostly sorta academic exercise for me personally), ...

Academic exercise or not, your mutex beats a CRITICAL_SECTION by an order of 
magnitude on a dual x86 in some queue tests I did.