Greetings.

After some years I think it's time to put on this topic again.

This patch series is an attempt to add a new thread model basing on the mcfgthread library 
(https://github.com/lhmouse/mcfgthread), which provides efficient implementations of mutexes, 
condition variables, once flags, etc. for native Windows.


The first patch is necessary because somewhere in libgfortran, `pthread_t` is referenced. If the 
thread model is not `posix`, it fails to compile.

The second patch implements `std::thread::hardware_concurrency()` for non-posix thread models. This 
would also work for the win32 thread model if `std::thread` would be supported in the future.

The third patch adds the `mcf` thread model for GCC and its libraries. A new builtin macro 
`__USING_MCFGTHREAD__` is added to indicate whether this new thread model is in effect. This grants 
`std::mutex` and `std::once_flag` trivial destructors; `std::condition_variable` is a bit 
unfortunate because its destructor is non-trivial, but in reality no cleanup is performed.


I have been bootstrapping GCC with the MCF thread model for more than five years. At the moment, C, 
C++ and Fortran are supported. Ada is untested because I don't know how to bootstrap it. Objective-C 
is not supported, because threading APIs for libobjc have not been implemented.

Please review. If there are any changes that I have to make, let me know.


-- 
Best regards,
LIU Hao