This is a bit old, but a couple of things jump out at me.
First, why are you showing us a Makefile? Are you saying that is what CMake generated? Or did you write a Makefile to call CMake?
Second, why are you overriding the CUDA compiler with clang++-20? I don’t expect your CUDA code to compile without using nvcc.
In general, the less you specify the better. Try a vanilla project that just defines the project and declares a target. Grow from there.