[boost] [fiber] Boost.Fiber 1.63.0 RC1 tries to use std::mutex which MinGW doesn't have

In both 1.62.0 and 1.63.0 RC1, building Boost.Fiber with my MinGW-w64 distro fails. That's because libstdc++ doesn't natively support the Clause 30 multithreading headers (std::mutex etc.) on Windows, and I make no attempt to build wrappers between the Windows and POSIX APIs (which is my personal choice). Boost's build system should detect this, and use the perfectly valid Boost.Thread implementation. Indeed, "C++11 mutex : no" is printed during the build's configuration, but Fiber's build fails anyways. My configuration output, and the errors emitted by a typical translation unit, are recorded here: https://github.com/StephanTLavavej/mingw-distro/issues/28 Thanks, STL