prefer OCaml 4.13 new String functions over Extlib's

.gitlab-ci.yml

@@ -15,7 +15,7 @@ default:
 
 variables:
   DEFAULT: "master"
-  OCAML: "4.11"
+  OCAML: "4.13"
 
 ################################################################################
 ### TESTS

meta_simplify.ml

@@ -28,7 +28,7 @@ open Cil_types
 *)
 let is_not_orig_variable lv =
   lv.lv_name = "__retres" ||
-  Extlib.string_prefix "__fc_" lv.lv_name ||
+  String.starts_with ~prefix:"__fc_" lv.lv_name ||
   match lv.lv_origin with
     | Some v ->
       not v.vsource || v.vtemp ||