HAVE_opencv_ocl implies HAVE_OPENCL, so checking for both is not
necessary.

2. Let Farneback to be the default optical flow method.
3. Fix a timing method bug for ocl path.
4. Remove useless finish operation in farneback
5. Refactor buffer usage.

Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.

Accuracy and performance tests are also provided.

Turns out, you can't use preprocessor directives inside macro arguments.
Who'd have thought?

Now it allows choosing between arbitrary implementation variants.

Backport from master.

command line option '-Wsign-promo' is valid for C++/ObjC++ but not for C [enabled by default]

* disabled warnings from thrust
* fixed warnings from ts_gtest.h
* possibly fixed superres compilation in Debug mode on Windows