Can't build OpenCV 3.1 on Windows (using MinGW) - opencv

Question

I need to build OpenCV from source, because I want to make modifications to it. So using pre-built binaries is not an option for me. I'm doing this on Windows 8. I do not have MS Visual Studio, and I don't want to use it.
Here is how I reproduce the build error:
I git clone https://github.com/opencv/opencv
I use CMake 3.7.1, press Configure, then press Generate. It seems to work (no errors). I named the output folder opencv/release.
I go to the opencv/release folder, and type mingw32-make.
Eventually, after compiling for a while, it displays the following error message:
Building CXX object modules/core/CMakeFiles/opencv_core.dir/src/parallel.cpp.obj
modules\core\src\parallel.cpp:135:43: error: operator '&&' has no right operand
modules\core\src\parallel.cpp: In function 'void cv::parallel_for_(const cv::Range&, const cv::ParallelLoopBody&, double)':
modules\core\src\parallel.cpp:311:52: error: 'parallel_for_pthreads' was not declared in this scope parallel_for_pthreads(range, body, nstripes);
^
modules\core\src\parallel.cpp: In function 'int cv::getNumThreads()':
modules\core\src\parallel.cpp:370:50: error: 'parallel_pthreads_get_threads_num' was not declared in this scope return parallel_pthreads_get_threads_num();
^
modules\core\src\parallel.cpp: In function 'void cv::setNumThreads(int)':
modules\core\src\parallel.cpp:431:46: error: 'parallel_pthreads_set_threads_num' was not declared in this scope parallel_pthreads_set_threads_num(threads);
^
modules\core\CMakeFiles\opencv_core.dir\build.make:990: recipe for target 'modules/core/CMakeFiles/opencv_core.dir/src/parallel.cpp.obj' failed mingw32-make[2]: *** [modules/core/CMakeFiles/opencv_core.dir/src/parallel.cpp.obj] Error 1
Note: I tried to compile both with and without TPP (in the CMake config), and I get the same error message.
Also, I read the other similar questions on StackOverflow and the answers do not help.