diff --git a/src/build.ml b/src/build.ml
index 4e859b1f..3214e366 100644
--- a/src/build.ml
+++ b/src/build.ml
@@ -122,6 +122,11 @@ let fanout3 a b c =
   (a &&& (b &&& c))
   >>>
   arr (fun (a, (b, c)) -> (a, b, c))
+let fanout4 a b c d =
+  let open O in
+  (a &&& (b &&& (c &&& d)))
+  >>>
+  arr (fun (a, (b, (c, d))) -> (a, b, c, d))
 
 let rec all = function
   | [] -> arr (fun _ -> [])
diff --git a/src/build.mli b/src/build.mli
index 96555071..ba950ec1 100644
--- a/src/build.mli
+++ b/src/build.mli
@@ -30,6 +30,7 @@ val second : ('a, 'b) t -> ('c * 'a, 'c * 'b) t
     The default definition may be overridden with a more efficient version if desired. *)
 val fanout  : ('a, 'b) t -> ('a, 'c) t -> ('a, 'b * 'c) t
 val fanout3 : ('a, 'b) t -> ('a, 'c) t -> ('a, 'd) t ->  ('a, 'b * 'c * 'd) t
+val fanout4 : ('a, 'b) t -> ('a, 'c) t -> ('a, 'd) t -> ('a, 'e) t -> ('a, 'b * 'c * 'd * 'e) t
 
 val all : ('a, 'b) t list -> ('a, 'b list) t
 
diff --git a/src/gen_rules.ml b/src/gen_rules.ml
index 47bad378..a906c98b 100644
--- a/src/gen_rules.ml
+++ b/src/gen_rules.ml
@@ -87,7 +87,7 @@ module Gen(P : Params) = struct
           fun x -> x
       in
       SC.add_rule sctx
-        (Build.fanout3
+        (Build.fanout4
            (dep_graph >>>
             Build.arr (fun dep_graph ->
               Ocamldep.names_to_top_closed_cm_files
@@ -98,21 +98,22 @@ module Gen(P : Params) = struct
                 (String_map.keys modules)))
            (SC.expand_and_eval_set sctx ~scope ~dir lib.c_library_flags ~standard:[])
            (Ocaml_flags.get flags mode)
+           (SC.expand_and_eval_set sctx ~scope ~dir lib.library_flags ~standard:[])
          >>>
          Build.run ~context:ctx (Dep compiler)
            ~extra_targets:(
              match mode with
              | Byte -> []
              | Native -> [lib_archive lib ~dir ~ext:ctx.ext_lib])
-           [ Dyn (fun (_, _, flags) -> As flags)
+           [ Dyn (fun (_, _, flags, _) -> As flags)
            ; A "-a"; A "-o"; Target target
            ; As stubs_flags
-           ; Dyn (fun (_, cclibs, _) -> Arg_spec.quote_args "-cclib" (map_cclibs cclibs))
-           ; As (List.map lib.library_flags ~f:(SC.expand_vars sctx ~scope ~dir))
+           ; Dyn (fun (_, cclibs, _, _) -> Arg_spec.quote_args "-cclib" (map_cclibs cclibs))
+           ; Dyn (fun (_, _, _, library_flags) -> As library_flags)
            ; As (match lib.kind with
                | Normal -> []
                | Ppx_deriver | Ppx_rewriter -> ["-linkall"])
-           ; Dyn (fun (cm_files, _, _) -> Deps cm_files)
+           ; Dyn (fun (cm_files, _, _, _) -> Deps cm_files)
            ]))
 
   let build_c_file (lib : Library.t) ~scope ~dir ~requires ~h_files c_name =
diff --git a/src/jbuild.ml b/src/jbuild.ml
index 52af9fae..14298b07 100644
--- a/src/jbuild.ml
+++ b/src/jbuild.ml
@@ -533,7 +533,7 @@ module Library = struct
     ; c_names                  : string list
     ; cxx_flags                : Ordered_set_lang.Unexpanded.t
     ; cxx_names                : string list
-    ; library_flags            : String_with_vars.t list
+    ; library_flags            : Ordered_set_lang.Unexpanded.t
     ; c_library_flags          : Ordered_set_lang.Unexpanded.t
     ; self_build_stubs_archive : string option
     ; virtual_deps             : string list
@@ -555,7 +555,7 @@ module Library = struct
        field_oslu "cxx_flags"                                                >>= fun cxx_flags                ->
        field      "c_names" (list string) ~default:[]                        >>= fun c_names                  ->
        field      "cxx_names" (list string) ~default:[]                      >>= fun cxx_names                ->
-       field      "library_flags" (list String_with_vars.t) ~default:[]      >>= fun library_flags            ->
+       field_oslu "library_flags"                                            >>= fun library_flags            ->
        field_oslu "c_library_flags"                                          >>= fun c_library_flags          ->
        field      "virtual_deps" (list string) ~default:[]                   >>= fun virtual_deps             ->
        field      "modes" Mode.Dict.Set.t ~default:Mode.Dict.Set.all         >>= fun modes                    ->
diff --git a/src/jbuild.mli b/src/jbuild.mli
index 3bf19fd1..a11147a6 100644
--- a/src/jbuild.mli
+++ b/src/jbuild.mli
@@ -138,7 +138,7 @@ module Library : sig
     ; c_names                  : string list
     ; cxx_flags                : Ordered_set_lang.Unexpanded.t
     ; cxx_names                : string list
-    ; library_flags            : String_with_vars.t list
+    ; library_flags            : Ordered_set_lang.Unexpanded.t
     ; c_library_flags          : Ordered_set_lang.Unexpanded.t
     ; self_build_stubs_archive : string option
     ; virtual_deps             : string list