[PATCH] Support thread cleanup without mingwm10.dll

[Danny Smith]

> OK, if noone has any strong feelings about retaining a 'singlethreaded mode'
> when GCC is built without --disable-threads, then I will update the
> documentation and make _MT always-on.
>

Sorry about the fat fingers.

I think that a runtime 'singlethreaded mode" should be available --
ie,  gthread_active_p()  returns false when thread API is not
referenced -- for users who do not need the overhead of thread-safety
in libstdc++.

Danny