added MinGW to CI and fixed local MinGW build

[firewave]

I removed LDFLAGS from the documentation since the Makefile sets it implicitly.

Somehow the MinGW setup in the CI contains which and uname which are not available if I install MinGW locally. So even though the build works in the CI we need to adjust the detection and configuration in the Makefile. These are more reasons which would justify dropping the Makefile in favor of CMake (but I am not prepared to do that for several reasons...yet).

This is in preparation of natively running daca@home on Windows via MinGW.

[firewave]

The MinGW x86 build fails with

cli/threadexecutor.cpp: In member function 'unsigned int ThreadExecutor::check()':
cli/threadexecutor.cpp:421:87: error: no matching function for call to 'async(std::launch, unsigned int (__attribute__((stdcall)) &)(ThreadExecutor*), ThreadExecutor*)'
             threadFutures.emplace_back(std::async(std::launch::async, threadProc, this));
                                                                                       ^
In file included from cli/threadexecutor.cpp:54:
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future:1712:5: note: candidate: 'template<class _Fn, class ... _Args> std::future<typename std::result_of<typename std::decay<_Tp>::type(typename std::decay<_Args>::type ...)>::type> std::async(std::launch, _Fn&&, _Args&& ...)'
     async(launch __policy, _Fn&& __fn, _Args&&... __args)
     ^~~~~
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future:1712:5: note:   template argument deduction/substitution failed:
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future: In substitution of 'template<class _Fn, class ... _Args> std::future<typename std::result_of<typename std::decay<_Tp>::type(typename std::decay<_Args>::type ...)>::type> std::async(std::launch, _Fn&&, _Args&& ...) [with _Fn = unsigned int (__attribute__((stdcall)) &)(ThreadExecutor*); _Args = {ThreadExecutor*}]':
cli/threadexecutor.cpp:421:87:   required from here
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future:1712:5: error: function returning a function
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future:1745:5: note: candidate: 'template<class _Fn, class ... _Args> std::future<typename std::result_of<typename std::decay<_Tp>::type(typename std::decay<_Args>::type ...)>::type> std::async(_Fn&&, _Args&& ...)'
     async(_Fn&& __fn, _Args&&... __args)
     ^~~~~
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future:1745:5: note:   template argument deduction/substitution failed:
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future: In substitution of 'template<class _Fn, class ... _Args> std::future<typename std::result_of<typename std::decay<_Tp>::type(typename std::decay<_Args>::type ...)>::type> std::async(_Fn&&, _Args&& ...) [with _Fn = std::launch; _Args = {unsigned int (__attribute__((stdcall)) &)(ThreadExecutor*), ThreadExecutor*}]':
cli/threadexecutor.cpp:421:87:   required from here
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/future:1745:5: error: no type named 'type' in 'class std::result_of<std::launch(unsigned int (__attribute__((stdcall)) *)(ThreadExecutor*), ThreadExecutor*)>'
mingw32-make: *** [Makefile:597: cli/threadexecutor.o] Error 1