Why is it in common to determine whether to compile some source code based on input values?

[jungyin]

I am trying to compile my demo, but it always prompts that the hpp file for OpenCV cannot be found. By searching for the cmake file, I believe it is due to the common.cmake process, which only imports OpenCV when compiling on am62, x86 targets, and x86 hosts. The other two situations do not occur. What is the reason for this?