Motivation for this change

Currently autoPatchelfHook fails as soon as any dependency cannot be satisfied. But in some cases it is favorable to continue patching and ignore the missing dependencies. Some binary distributions contain optional features with their own dependencies. The current autoPatchelfHook forces you to patch all dependencies even if you know you will never use those features.
A specific example would be the python wheel distribution of Pytorch which contains files which depend on cuda, but in practice this dependency doesn't need to be satisfied if you don't plan to use cuda. (pytorch/pytorch#38624)

To solve this problem, I added the possibility to ignore missing dependencies by setting the environment variable autoPatchelfIgnoreMissing.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.