pack+package: workaround ocamlbuild#272

ocamlbuild should pass -package(...) flags to ocamlfind when building
a -pack-ed file, see

  ocaml/opam-repository#11628 (comment)

myocamlbuild.ml

@@ -1,5 +1,13 @@
 open Ocamlbuild_plugin
 
+let ocamlfind_and_pack = function
+  | After_rules ->
+     if !Options.use_ocamlfind then
+       pflag ["ocaml"; "pack"] "package"
+         (fun pkg -> S [A "-package"; A pkg]);
+  | _ -> ()
+
 let () = dispatch Ocb_stubblr.(
   init & ccopt ~tags:["accelerate"] "-DACCELERATE -msse2 -maes"
+  & ocamlfind_and_pack
 )