The opam patch makes ocaml link dependent executables with
-L${LLVM_OCAML_EXTERNAL_LLVM_LIBDIR}. This variable was previously
undefined and as a result the linker would previously be called with
just -L which makes it ignore the next argument. This would lead strange
linking errors, like missing caml_apply2.

Despite defining this variable correctly, propagating llvm is still
necessary for linking to complete. In case ocaml-llvm is a transitive
dependency only, propagatedBuildInputs is not enough. To avoid having to
guess which version of llvm was used, we provide the right one in
passthrough.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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.

Notify maintainers

cc @vbgl