opencv: Cannot build OpenCV with Intel VA, asking for "CL/va_ext.h" file

System information (version)

• OpenCV => 4.4.0
• Operating System / Platform => Alpine 3.12
• Compiler => clang++

Detailed description

I am trying to compile opencv with VA_INTEL. I have compiled and installed Intel MSDK and OpenCL. But while compiling, the following error shows up

/tmp/opencv-4.3.0/modules/core/src/va_intel.cpp:29:12: fatal error: 'CL/va_ext.h' file not found
#  include <CL/va_ext.h>
           ^~~~~~~~~~~~~
[ 30%] Building CXX object modules/core/CMakeFiles/opencv_core.dir/stat.avx2.cpp.o
1 error generated.
make[2]: *** [modules/core/CMakeFiles/opencv_core.dir/build.make:1176: modules/core/CMakeFiles/opencv_core.dir/src/va_intel.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:1464: modules/core/CMakeFiles/opencv_core.dir/all] Error 2
make: *** [Makefile:183: all] Error 2

This has something to do with the fact its looking for a file va_ext.h in the include/CL directory it would seem; as specified here https://github.com/opencv/opencv/blob/master/modules/core/src/va_intel.cpp#L29.

The issue is, I can’t find this file anywhere. Neither do I know how to install it. When OpenCL headers is installed, it doesn’t come with this file. So not sure what I am doing wrong, as I have been stuck on this for so long.

Steps to reproduce

compile opencv with -D WITH_VA_INTEL=ON

Issue submission checklist

• I report the issue, it’s not a question
• I checked the problem with documentation, FAQ, open issues, answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc