diff --git a/vendors/ocaml-lmdb/config/discover.ml b/vendors/ocaml-lmdb/config/discover.ml
index bc91a1571..1b0496ec9 100644
--- a/vendors/ocaml-lmdb/config/discover.ml
+++ b/vendors/ocaml-lmdb/config/discover.ml
@@ -1,4 +1,8 @@
 let () =
   let oc = open_out "c_flags.sexp" in
-  output_string oc (if Sys.word_size = 32 then "(-DMDB_VL32)" else "()") ;
+  let w = "-W -Wall -Wno-unused-parameter -Wbad-function-cast -Wuninitialized" in
+  let thread = "-pthread" in
+  let opt = "-O2 -g" in
+  Printf.fprintf oc "(%s %s %s %s)" w thread opt
+    (if Sys.word_size = 32 then "-DMDB_VL32" else "") ;
   close_out oc