ocaml bindings + ocamlbuild problem


I'm trying to use Llvm_bitwriter + ocamlbuild, but it doesn't
recognize its dependency on Unix, so it doesn't compile. Minimal

(* FILE: minimal.ml *)

let main () =
  let m = Llvm.create_module (Llvm.global_context ()) "test" in
    ignore (Llvm_bitwriter.output_bitcode stdout m)

main ()

(* FILE: _tags *)

<*.{byte,native}>: g++, use_unix, use_llvm, use_llvm_bitwriter

(* FILE: myocamlbuild.ml *)

open Ocamlbuild_plugin;;

ocaml_lib ~extern:true "llvm";;
ocaml_lib ~extern:true "llvm_bitwriter";;

flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;

I try to compile with the command line

ocamlbuild minimal.native

and get the result:

Finished, 1 target (0 cached) in 00:00:02.
+ /usr/local/bin/ocamlopt.opt llvm.cmxa llvm_bitwriter.cmxa -cc g++ unix.cmxa minimal.cmx -o minimal.native
File "_none_", line 1, characters 0-1:
Error: No implementations provided for the following modules:
         Unix referenced from /usr/local/lib/ocaml/llvm_bitwriter.cmxa(Llvm_bitwriter)
Command exited with code 2.
Compilation unsuccessful after building 4 targets (0 cached) in 00:00:01.

(note that unix.cmxa appears after llvm_bitwriter.cmxa). However, if I try

ocamlopt.opt -cc g++ unix.cmxa llvm.cmxa llvm_bitwriter.cmxa minimal.ml

it compiles fine (note that unix.cmxa is now listed BEFORE llvm_bitwriter.cmxa).

Is there any way to force unix.cmxa be put first in the compilation
command line? I would really prefer not to have to write a Makefile...