Error when compiling OpenCV 4.0 with CUDA 8

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Hi, I'm using an older Q5000 GPU in my machine that needs CUDA 8. It was tricky enough to get that to run on Ubuntu 18.04, switching to an older GCC etc but now I run into another error compiling OpenCV 4:

 /home/b/opencv_contrib/modules/cudaimgproc/src/cuda/clahe.cu(191): error: identifier "__shfl_down" is undefined

/home/b/opencv_contrib/modules/cudaimgproc/src/cuda/clahe.cu(210): error: identifier "__shfl_down" is undefined

/home/b/opencv_contrib/modules/cudaimgproc/src/cuda/clahe.cu(236): error: identifier "__shfl_down" is undefined

/home/b/opencv_contrib/modules/cudaimgproc/src/cuda/clahe.cu(255): error: identifier "__shfl_down" is undefined

Is CUDA 8 too old for OpenCV 4? :/