Commit 4cd63de broke CUDA_RUNTIME_LIB variable

[overvalidated]

In this commit variable was removed but is referenced in warn_in_case_of_duplicates, determine_cuda_runtime_lib_path causing errors.

[TimDettmers]

I was working on a new release and introduced this bug since we currently do not have tests for the CUDA setup. I fixed the bug 659a7df and preparing a post-release which should be uploaded within a few minutes. Thank you for your timely bug report!