Use more standard command line options for ppx drivers

src/gen_rules.ml

@@ -714,9 +714,9 @@ module Gen(P : Params) = struct
                (Dep ppx_exe)
                [ Dyn (specific_args_for_ppx_rewriters ~dir ~lib_name)
                ; As flags
-               ; A "-dump-ast"
+               ; A "--dump-ast"
                ; A "-o"; Target dst
-               ; Ml_kind.flag kind; Dep src
+               ; Ml_kind.ppx_driver_flag kind; Dep src
                ])
         )
     )

src/ml_kind.ml

@@ -12,6 +12,8 @@ let to_string = choose "impl" "intf"
 
 let flag t = choose (Arg_spec.A "-impl") (A "-intf") t
 
+let ppx_driver_flag t = choose (Arg_spec.A "--impl") (A "--intf") t
+
 let ext = choose ".ml" ".mli"
 
 module Dict = struct

src/ml_kind.mli

@@ -10,6 +10,7 @@ val to_string : t -> string
 val ext : t -> string
 
 val flag : t -> _ Arg_spec.t
+val ppx_driver_flag : t -> _ Arg_spec.t
 
 module Dict : sig
   type kind = t