modified OpenCL SURF API and the tests in 2.4.x to prove that it gives different from CPU results
made SURF_OCL default constructor parameters the same as SURF
modified SURF's performance test to test OpenCL version as well
fixed doc builder warnings; make sure the tests give reasonable results when OpenCL is not available
making OCL tests conform to the comon style
disabling failing tests
fixing check_docs2.py issue
fixing conditional compilation
disabling calls to SURF_OCL causing tests failures
splitting plain and OCL tests for SURF.
Merge pull request #2527 from apavlenko:ocl_surf_24