NVCC's default architecture may be newer than the one supported by the
machine's GPU.
In such cases it's useful to have an environment variable for initializing
CMAKE_CUDA_ARCHITECTURES to avoid specifying it for every invocation.
Simplifies CUDA target architecture handling.
Required for Clang support as Clang doesn't automatically select a supported architecture.
We detect a supported architecture during compiler identification and set CMAKE_CUDA_ARCHITECTURES to it.
Introduces CMP0104 for backwards compatibility with manually setting code generation flags with NVCC.
Implements #17963.
