Cuda woes: gnu++14 and cudnnGetConvolutionForwardAlgorithm

[mratsim]

Since Nim 1.2 Nim now adds -std:gnu++14 to the C++ target which we cannot avoid even after nim-lang/Nim#13933 because the "override" only reorders the arguments passed to the compiler.
Even using clang doesn't prevent that gnu++14.

This can be worked around by using Nim 1.0.4 which leads us to the second problem.

Also since CuDNN 8 the procedure cudnnGetConvolutionForwardAlgorithm has been removed. It can be found under the symbol cudnnGetConvolutionForwardAlgorithm_v7 for the time being, but this requires nimcuda to be updated.