Replace `CUDA_VERSION` with `cudaRuntimeGetVersion()` for `if`/`IF` conditionals

This is needed to support CUDA Python as CUDA_VERSION in CUDA Python does not mean the current CUDA version.

[kmaehashi]

Let’s prefix all compile_time_envs with CUPY_ to avoid confusion and conflict like this 😅 https://github.com/cupy/cupy/blob/master/cupy_setup_build.py#L947-L957

[kmaehashi]

Let me clarify that driver.get_build_version() returns CUDA Python’s CUDA_VERSION if it is left as it-is (when CUDA Python is enabled).

Yes.

So I think we need to change driver.get_build_version() somehow to see cuda.h’s CUDA_VERSION.

I think it’s better to expose CUDA_VERSION as cupy_backends.cuda.libs.cusolver._get_cuda_build_version() etc.