diff --git a/vendor/spawn/src/dune b/vendor/spawn/src/dune
index cbf126be026a..9a1f8620819b 100644
--- a/vendor/spawn/src/dune
+++ b/vendor/spawn/src/dune
@@ -2,6 +2,14 @@
  (name spawn)
  (foreign_stubs
   (language c)
-  (flags :standard -DUNICODE -D_UNICODE)
+  (flags (:standard (:include flags.sexp)))
   (names spawn_stubs))
  (libraries unix threads.posix))
+
+(rule
+  (with-stdout-to flags.sexp
+    (run ocaml %{dep:flags.ml})))
+
+(rule
+  (with-stdout-to flags.ml
+    (echo "print_endline @@ if Sys.win32 then \"(-DUNICODE -D_UNICODE)\" else \"()\"")))