In the following scenario (with 3.18 policies):
1. A CXX target is created.
2. CUDA language is enabled.
CMake 3.18 introduced CMP0104, which requires CUDA_ARCHITECTURES to be
set. Because the CXX target was created before CUDA was enabled it
wouldn't have it set. The Visual Studio generator would however end up
computing CUDA compile options for the CXX target, which would result in
a fatal error due to the policy violation.
There doesn't seem to be a reason to do this for targets that don't
actually use the CUDA language, so we can skip and generate the CXX
target just fine.
Fixes: #21341
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.
Raul Tambre