Seems I can't "unforget" erroneously forgotten files, so will have to forget them and then re-add them
#ifndef BOOST_THREAD_MUTEX_HPP
#define BOOST_THREAD_MUTEX_HPP
// mutex.hpp
//
// (C) Copyright 2007 Anthony Williams
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
#include <boost/thread/detail/platform.hpp>
#if defined(BOOST_THREAD_PLATFORM_WIN32)
#include <boost/thread/win32/mutex.hpp>
#elif defined(BOOST_THREAD_PLATFORM_PTHREAD)
#include <boost/thread/pthread/mutex.hpp>
#else
#error "Boost threads unavailable on this platform"
#endif
#endif