Sphericart depends on threads even when OpenMP is off

[frostedoyster]

Due to https://github.com/metatensor/gpu-lite/blob/6c221167aeca7e113ea6f27e9a2e12391bdb189a/CMakeLists.txt#L17 in GPUlite. Not sure what the best solution is (disable GPUlite in CMake if CUDA not requested, remove that line from GPUlite, etc.).
@Luthaf @RMeli

[Luthaf]

Why is this an issue? We can maybe add some kind of "don't use threads" preprocessor option to gpulite, but I'm not sure what this would gain us.