ALL: Include and use a copy of mingw-std-threads

This allows us to use threads and mutexes from C++11 on older MinGW
platforms.

AUTHORS

@@ -121,6 +121,15 @@ UTF-8 CPP
    external/utf8cpp/utf8.h or <https://www.boost.org/users/license.html>
    for details.
 
+mingw-std-threads
+*****************
+   xoreos uses and includes the header-only mingw-std-threads library
+   (<https://github.com/meganz/mingw-std-threads/>) as an alternative to
+   libwinpthread for C++11 std::thread in MinGW builds. mingw-std-threads
+   is licensed under the terms of the 2-clause BSD license. For the full
+   text, see external/mingw-std-threads/LICENSE or
+   <https://github.com/meganz/mingw-std-threads/blob/master/LICENSE>.
+
 GLM
 ***
    xoreos uses the header-only C++ mathematics library for graphics software

external/mingw-std-threads/LICENSE

@@ -0,0 +1,24 @@
+Copyright (c) 2016, Mega Limited
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+

external/mingw-std-threads/README.md

@@ -0,0 +1,41 @@
+mingw-std-threads
+=================
+
+Implementation of standard C++11 threading classes, which are currently still missing on MinGW GCC.
+
+Target Windows version
+----------------------
+This implementation should work with Windows XP (regardless of service pack), or newer.
+The library automatically detects the version of Windows that is being targeted (at compile time), and selects an implementation that takes advantage of available Windows features.
+In MinGW GCC, the target Windows version may optionally be selected by the command-line option `-D _WIN32_WINNT=...`.
+Use `0x0600` for Windows Vista, or `0x0601` for Windows 7.
+See "[Modifying `WINVER` and `_WIN32_WINNT`](https://docs.microsoft.com/en-us/cpp/porting/modifying-winver-and-win32-winnt)" for more details.
+
+Usage
+-----
+
+This is a header-only library. To use, just include the corresponding `mingw.xxx.h file`, where `xxx` would be the name of the standard header that you would normally include.
+
+For example, `#include "mingw.thread.h"` replaces `#include <thread>`.
+
+Compatibility
+-------------
+
+This code has been tested to work with MinGW-w64 5.3.0, but should work with any other MinGW version that has the `std` threading classes missing, has C++11 support for lambda functions, variadic templates, and has working mutex helper classes in `<mutex>`.
+
+Switching from the win32-pthread based implementation
+-----------------------------------------------------
+It seems that recent versions of MinGW-w64 include a Win32 port of pthreads, and have the `std::thread`, `std::mutex`, etc. classes implemented and working based on that compatibility
+layer.
+That is a somewhat heavier implementation, as it relies on an abstraction layer, so you may still want to use this implementation for efficiency purposes.
+Unfortunately you can't use this library standalone and independent of the system `<mutex>` headers, as it relies on those headers for `std::unique_lock` and other non-trivial utility classes.
+In that case you will need to edit the `c++-config.h` file of your MinGW setup and comment out the definition of _GLIBCXX_HAS_GTHREADS.
+This will cause the system headers not to define the actual `thread`, `mutex`, etc. classes, but still define the necessary utility classes.
+
+Why MinGW has no threading classes 
+----------------------------------
+It seems that for cross-platform threading implementation, the GCC standard library relies on the gthreads/pthreads library.
+If this library is not available, as is the case with MinGW, the classes `std::thread`, `std::mutex`, `std::condition_variable` are not defined.
+However, various usable helper classes are still defined in the system headers.
+Hence, this implementation does not re-define them, and instead includes those headers.
+