State we need a recent enough version of Core

 + Thanks to @damiendoligez for reporting the issue at
 https://github.com/ocaml/opam-repository/pull/5904.

opam

@@ -17,7 +17,7 @@ remove: ["ocamlfind" "remove" "ocl"]
 depends: [
   "atdgen"
   "base-threads"
-  "core"
+  "core" {>= "112.35.00"}
   "textutils"
   "re2"
   "ocamlfind" {build}