Prefer Windows threads to pthreads

[Marc-Aldorasi-Imprivata]

On mingw ASIO can only detect pthreads if the pthread header is included before it. Since mingw can use both pthreads and Windows threads, if you have multiple source files that access an io_context and only some of them include pthreads before asio then they can use the wrong thread type and corrupt memory. If asio instead prefers Windows threads to pthreads there will be no type confusion and no memory corruption.

The following program demonstrates the difference:

#if INCLUDE_PTHREAD
#include <pthread.h>
#endif
#include <asio.hpp>
#include <iostream>

int main() {
	std::cout << "asio::detail::thread is " << typeid(asio::detail::thread).name() << '\n';
}

When compiled with -DINCLUDE_PTHREAD it prints "asio::detail::thread is N4asio6detail12posix_threadE", but when compiled without that define prints "asio::detail::thread is N4asio6detail10win_threadE". With this patch it always prints "asio::detail::thread is N4asio6detail10win_threadE".