diff --git a/src/configurator/flags/jbuild b/src/configurator/flags/jbuild
new file mode 100644
index 00000000..68be2f39
--- /dev/null
+++ b/src/configurator/flags/jbuild
@@ -0,0 +1,6 @@
+(jbuild_version 1)
+
+(executable
+ ((name mk)))
+
+(rule (with-stdout-to flags.sexp (run ./mk.exe -ocamlv ${ocaml_version})))
diff --git a/src/configurator/flags/mk.ml b/src/configurator/flags/mk.ml
new file mode 100644
index 00000000..4c72c580
--- /dev/null
+++ b/src/configurator/flags/mk.ml
@@ -0,0 +1,25 @@
+open Printf
+
+let parse_version s =
+  Scanf.sscanf s "%d.%d.%d" (fun a b c -> a, b, c)
+
+let () =
+  let usage =
+    sprintf "%s -ocamlv version" (Filename.basename Sys.executable_name) in
+  let ocaml_version = ref "" in
+  let anon _ =
+    raise (Arg.Bad "anonymous arguments aren't accepted") in
+  Arg.parse
+    [ "-ocamlv"
+    , Arg.String (fun s -> ocaml_version := s)
+    , "Version of ocaml being used"
+    ] anon usage;
+  if !ocaml_version = "" then
+    raise (Arg.Bad "Provide version with -ocamlv")
+  else
+    let (x, y, _) = parse_version !ocaml_version in
+    if x >= 4 && y > 2 then (
+      printf "()\n"
+    ) else (
+      printf "(-w -50)\n"
+    )
diff --git a/src/configurator/jbuild b/src/configurator/jbuild
index 03b0c4fd..2b696467 100644
--- a/src/configurator/jbuild
+++ b/src/configurator/jbuild
@@ -1,7 +1,7 @@
 (library
  ((name configurator)
-  (flags (:standard -safe-string))
   (libraries (stdune ocaml_config))
+  (flags (:standard -safe-string (:include flags/flags.sexp)))
   (preprocess no_preprocessing)))
 
 (jbuild_version 1)
diff --git a/src/stdune/caml/caml.ml b/src/stdune/caml/caml.ml
index a050ef96..82dc6ca9 100644
--- a/src/stdune/caml/caml.ml
+++ b/src/stdune/caml/caml.ml
@@ -1,5 +1,6 @@
 module Filename = Filename
 module String   = String
+module Char     = Char
 module Result   = Result
 
 type ('a, 'error) result = ('a, 'error) Result.t =
diff --git a/src/stdune/char.ml b/src/stdune/char.ml
new file mode 100644
index 00000000..a6c6b1ee
--- /dev/null
+++ b/src/stdune/char.ml
@@ -0,0 +1,9 @@
+module Char = Caml.Char
+
+include struct
+  [@@@warning "-32-3"]
+  let uppercase_ascii    = Char.uppercase
+  let lowercase_ascii    = Char.lowercase
+end
+
+include Char