simplify compilation

src/dune

@@ -3,16 +3,14 @@
   (public_name opam-file-format)
   (synopsis "Parser and printer for the opam file syntax")
   (wrapped false)
-  (modules :standard \ flags)
   (flags :standard (:include flags.sexp)))
 
 (rule
-  (with-stdout-to flags.ml
-    (echo "print_string (if String.sub Sys.ocaml_version 0 5 = \"4.02.\" then \"(-w -50)\" else \"()\")")))
-
+ (enabled_if (< %{ocaml_version} "4.03"))
+ (action (with-stdout-to flags.sexp (echo "(-w -50)"))))
 (rule
-  (with-stdout-to flags.sexp
-    (run ocaml %{dep:flags.ml})))
+ (enabled_if (>= %{ocaml_version} "4.03"))
+ (action (with-stdout-to flags.sexp (echo "()"))))
 
 (ocamlyacc opamBaseParser)
 (ocamllex opamLexer)