make moonpool.forkjoin optional (only on OCaml >= 5.0)

src/forkjoin/dune

@@ -5,8 +5,8 @@
  (public_name moonpool.forkjoin)
  (synopsis "Fork-join parallelism for moonpool")
  (flags :standard -open Moonpool)
- (preprocess
-  (action
-   (run %{project_root}/src/cpp/cpp.exe %{input-file})))
+ (optional)
+ (enabled_if
+  (>= %{ocaml_version} 5.0))
  (libraries
    moonpool moonpool.private))

src/forkjoin/moonpool_forkjoin.ml

@@ -1,5 +1,3 @@
-[@@@ifge 5.0]
-
 module A = Moonpool.Atomic
 module Suspend_ = Moonpool.Private.Suspend_
 module Domain_ = Moonpool_private.Domain_
@@ -218,5 +216,3 @@ let map_list ?chunk_size f (l : _ list) : _ list =
       match res.(i) with
       | None -> assert false
       | Some x -> x)
-
-[@@@endif]

src/forkjoin/moonpool_forkjoin.mli

@@ -4,8 +4,6 @@
 
     @since 0.3 *)
 
-[@@@ifge 5.0]
-
 val both : (unit -> 'a) -> (unit -> 'b) -> 'a * 'b
 (** [both f g] runs [f()] and [g()], potentially in parallel,
     and returns their result when both are done.
@@ -105,5 +103,3 @@ val map_list : ?chunk_size:int -> ('a -> 'b) -> 'a list -> 'b list
 (** [map_list f l] is like [List.map f l], but runs in parallel.
     @since 0.3
     {b NOTE} this is only available on OCaml 5. *)
-
-[@@@endif]