arrayfire: update

- Depend on `clfft`, `clblas`, and `opencl-clhpp` from nixpkgs (do not
fetch their sources)
- Add `buildDocs` flag. When set to true, arrayfire's docs will be
built using `doxygen`.
- Ensure that `cudatoolkit` is not evaluated on darwin.
- Move `python` from `buildInputs` to `nativeBuildInputs`.
- Meta: Shrink `description` and add `longDescription`.
- Meta: `homepage` formatting now adheres to nixpkgs manual.
- Meta: remove version
- A few minour syntactic-only changes.