Use backslashes when passing staged pps driver on Windows

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

src/dune_rules/pp_spec_rules.ml

@@ -281,6 +281,16 @@ let pp_one_module
                ~dir:(Super_context.context sctx |> Context.build_dir |> Path.build)
                (S [ Dep (Path.build exe); As driver_flags; As flags ]))
         in
+        let args =
+          match args with
+          | exe :: args when Sys.win32 ->
+            (* On Windows, [exe] is executed by the compiler using
+               [Sys.command], ie [cmd.exe].  Thus, its path must contain
+               backslashes only, as forward slashes are used to introduce
+               options, eg "dir/?". *)
+            String.concat ~sep:"\\" (String.split_on_char ~sep:'/' exe) :: args
+          | _ -> args
+        in
         [ "-ppx"; String.quote_list_for_shell args ]
       in
       let pp =