Changes

Define NPY_OS_MINGW on Windows/MinGW. (#28)

  - There is a small bug in numpy's f2yc which does not detect MinGW
    environments as it is supposed to.

    See: https://github.com/numpy/numpy/issues/24761

  - This make file is used for MinGW only. So we can just define the
    variable without any other checks being needed.

    This also needs a CMake configuration or at least the Makefile should
    be improved a bit.