Increase max policy range for CMake to 3.18

[drbenmorgan]

CMake 3.18 introduced policy CMP0104 on the initialization of the new CMAKE_CUDA_ARCHITECTURES variable and subsequent use of code generation flags. Without this policy being set to NEW, warnings are emitted at CMake generation time about CUDA_ARCHITECTURES not being set for targets.

Bump max CMake policy range to 3.18 to set a default for CMAKE_CUDA_ARCHITECTURES if not supplied by the user.

This can still result in nvlink errors when linking vecgeom_static if that was compiled with architectures incompatible(?) with that for Celeritas. Not sure how to check that yet.

[sethrj]

Ahhh, so bumping the policy is what automatically sets it. I'd wondered about that. Thanks! It looks like it detects our old K40 as having 3.0 instead of 3.5, but still that's a nice feature to have :)

[drbenmorgan]

Ahhh, so bumping the policy is what automatically sets it. I'd wondered about that. Thanks! It looks like it detects our old K40 as having 3.0 instead of 3.5, but still that's a nice feature to have :)

Yes, I had this issue on my CUDA 10.2/Quadro P400 setup. That I think led to the VecGeom undefined references, as that (from Spack) was built with cuda_arch=35?

[sethrj]

Could be -- I think the spack environment file in the repo is constructed for our older test machine (which has version 3.5).