fix(build): Add std threading support in Windows build

Use posix versions of mingw toolchains, which say:
> This package contains the C++ compiler, supporting cross-compiling to
64-bit MinGW-w64 targets, using the POSIX threading model.

Fix build failure saying that std::mutex was not declared

windows/cross-compile/build.sh

@@ -152,11 +152,15 @@ then
   apt-get install -y --no-install-recommends \
                     g++-mingw-w64-i686 \
                     gcc-mingw-w64-i686
+  update-alternatives --set i686-w64-mingw32-gcc /usr/bin/i686-w64-mingw32-gcc-posix
+  update-alternatives --set i686-w64-mingw32-g++ /usr/bin/i686-w64-mingw32-g++-posix
 elif [[ "$ARCH" == "x86_64" ]]
 then
   apt-get install -y --no-install-recommends \
                     g++-mingw-w64-x86-64 \
                     gcc-mingw-w64-x86-64
+  update-alternatives --set x86_64-w64-mingw32-gcc /usr/bin/x86_64-w64-mingw32-gcc-posix
+  update-alternatives --set x86_64-w64-mingw32-g++ /usr/bin/x86_64-w64-mingw32-g++-posix
 fi