Improve the behavior of "jbuilder exec" on Windows

bin/main.ml

@@ -757,8 +757,7 @@ let exec =
   let go common context prog args =
     set_common common ~targets:[];
     let log = Log.create () in
-    Future.Scheduler.go ~log
-      (Main.setup ~log common >>= fun setup ->
+    let setup = Future.Scheduler.go ~log (Main.setup ~log common) in
     let context =
       match List.find setup.contexts ~f:(fun c -> c.name = context) with
       | Some ctx -> ctx
@@ -774,11 +773,19 @@ let exec =
     | Some real_prog ->
       let real_prog = Path.to_string real_prog     in
       let env       = Context.env_for_exec context in
+      let argv      = Array.of_list (prog :: args) in
       if Sys.win32 then
-           Future.run ~env Strict real_prog args
+        let pid =
+          Unix.create_process_env real_prog argv env
+            Unix.stdin Unix.stdout Unix.stderr
+        in
+        match snd (Unix.waitpid [] pid) with
+        | WEXITED   0 -> ()
+        | WEXITED   n -> exit n
+        | WSIGNALED _ -> exit 255
+        | WSTOPPED  _ -> assert false
       else
-           Unix.execve real_prog (Array.of_list (prog :: args)) env
-      )
+        Unix.execve real_prog argv env
   in
   ( Term.(const go
           $ common