CudaExec policy undefined when building with CUDA but w/o RAJA

[szellmann]

I think here: here and here additional checks are required for if ASCENT_RAJA_ENABLED is defined. CudaExec is only defined if ASCENT_RAJA_ENABLED is defined. I'd submit a PR, but I'm not sure if, conversely, the bug is that CudaExec should be defined in either case (with or without RAJA). Adding additional checks there gets me passed this issue though. Without them, files including that header won't compile.

[cyrush]

@szellmann thanks for reporting this -- the CudaExec policy does require RAJA.

There are are a few more places that need better guards, I will take a pass and try to remedy all of these.