[llvm] [offload][runtimes] Forward user-provided system configuration. (PR #96303)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> [3de162f](https://github.com/llvm/llvm-project/commit/3de162fee44376b3857c73958d656f76b17e50f5) sets it to `CUDAToolkit_BIN_DIR`, but the meaning it previously had (the path passed to `clang --cuda-path`) would be `CUDAToolkit_LIBRARY_ROOT`.

No, it takes the directory of `opt/cuda/bin` which is `opt/cuda` for example.

https://github.com/llvm/llvm-project/pull/96303