If device does not support double, returns immediately.
Double floating points are not default supported by OpenCL.
Update the OpenCL documents.
fix three warning about mog, mog2 and KNearestNeighbour.
fix warning about addWeighted and remove magnitudeSqr().
pr #1464 reverted as breaking the build for Linux without libv4l library.
fix for bug #3291:

add INSTALL command for TBB target
fixed ocl::minMax
fixed ocl::countNonZero
Missing closedir with opendir
Add opencl svm.
fix build error
Fix new blank line at EOF.
Make libv4l support optional for testing purposes.
fixed ocl::sum, ocl::sqrSum, ocl::absSum
fixed ocl::phase
fixed ocl::setIdentity
fixed ocl::meanStddev and created accuracy test for this function
perf test: update error message about relative error
fixed and extended ocl::norm
perf tests: added SANITY_CHECK_NOTHING()