Disable unused features in OpenCV build

We actually only require a very basic OpenCV setup. Disabling
the unused features speeds up the OpenCV build considerably.

README.win32

@@ -101,7 +101,7 @@ IMPORTANT: Get the right MinGW version!
 
 	mkdir build
 	cd build
-	cmake .. -G "MinGW Makefiles" -DBUILD_SHARED_LIBS=0
+	cmake .. -G "MinGW Makefiles" -DBUILD_DOCS=0 -DBUILD_PACKAGE=0 -DBUILD_PERF_TESTS=0 -DBUILD_SHARED_LIBS=0 -DBUILD_TESTS=0 -DBUILD_opencv_apps=0 -DBUILD_opencv_calib3d=0 -DBUILD_opencv_contrib=0 -DBUILD_opencv_features2d=0 -DBUILD_opencv_flann=0 -DBUILD_opencv_gpu=0 -DBUILD_opencv_java=0 -DBUILD_opencv_legacy=0 -DBUILD_opencv_ml=0 -DBUILD_opencv_nonfree=0 -DBUILD_opencv_objdetect=0 -DBUILD_opencv_ocl=0 -DBUILD_opencv_photo=0 -DBUILD_opencv_python=0 -DBUILD_opencv_stitching=0 -DBUILD_opencv_superres=0 -DBUILD_opencv_ts=0 -DBUILD_opencv_video=0 -DBUILD_opencv_videostab=0 -DBUILD_opencv_world=0 -DWITH_FFMPEG=0 -DWITH_JASPER=0 -DWITH_OPENEXR=0 -DWITH_TIFF=0
 
 4. Build and install OpenCV