Require posix thread model for Windows builds

VLC uses its own threading code on Windows. In other words we don't use pthread. But a lot of contribs use it. This is becoming a problem with libraries using C++11 std::thread.

On Windows it's only supported by gcc and clang when using the 'posix' thread model. It means they are using their pthread-based implementation and rely on winpthreads.

Not having C++ thread support on all platforms means we can't update some C++ based libraries like protobuf. It is also likely needed in Qt6.