Fix the following error for ocl::getOpenCLPlatforms() on Ubuntu 12.04 with gcc 4.8
OpenCV Error: Unknown error code -6 (OpenCL function is not available: [clGetPlatformIDs]) in opencl_check_fn, file /home/ahb/software/opencv/modules/ocl/src/cl_runtime/cl_runtime.cpp, line 83

The issue results from modules/ocl/src/cl_runtime/cl_runtime.cpp checking for
"linux" instead of "__linux__" (cp.  http://sourceforge.net/p/predef/wiki/OperatingSystems/)

Adjust all other occurrences of "defined(linux)" as well.
Merge pull request #2145 from andreasBihlmaier:2.4
Fixed cvtColor alpha channel docs
Merge pull request #2143 from Daniil-Osokin:fix_docs_cvt_color_alpha_channel