You’re asking the find command to search for files in the . and 02 directories and 02 does not exist.
CMake is a build tool and one of the key features is it can find libraries for you. It’s not necessary to have a script to find Lensfun because CMake can do it. darktable probably does extra stuff, like looking for Lensfun in non-standard locations or performing additional checks to see if Lensfun has everything darktable needs. You’ll have to look inside the file if you want to know exactly what it does.
A clean build means deleting the build directory, which is what you have done. I skimmed through one of your threads (this one: Fatal error in build of 4.6.1 - I don't understand how to fix). I suspect there are still some lingering files, directories, and/or configurations from Lensfun 0.3.95 which is causing problems. The documentation for CMake says it uses pkg-config to locate library files. pkg-config gets its data from .pc files that come with library packages. It’s worth a try to search for lensfun.pc files on your computer. This pkg-config guide may indicate where such files are located: Guide to pkg-config