Restore compatibility with OCaml 4.14 (rescript-lang#6926)

Author: cknitt

jscomp/ext/dune

@@ -3,7 +3,8 @@
  (wrapped false)
  (preprocess
   (action
-   (run %{bin:cppo} %{env:CPPO_FLAGS=} %{input-file})))
+   (run %{bin:cppo} -V OCAML:%{ocaml_version} %{env:CPPO_FLAGS=}
+     %{input-file})))
  (flags
   (:standard -w +a-4-42-40-9-48-70))
  (foreign_stubs

jscomp/ext/js_reserved_map.ml

@@ -23,7 +23,16 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. *)
 
 module STbl = struct
-  include Hashtbl.Make (String)
+  #if OCAML_VERSION >= (5, 0, 0)
+    include Hashtbl.Make (String)
+  #else
+    module StringHash : Hashtbl.HashedType with type t = string = struct
+      type t = string
+      let equal = String.equal
+      let hash = Hashtbl.hash (* polymorphic hash function *)
+    end
+    include Hashtbl.Make (StringHash)
+  #endif
 
   let of_array arr =
     let tbl = create (Array.length arr) in