ocamlPackages.llvm: propagate needed libs, fix missing build parameter

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.