Drop -warn-error from flags
It breaks the build on OCaml 4.13 because of new warnings:
ocamlc.opt -g -dtypes -w s-3+52 -c -warn-error A-52 varcc.ml
File "varcc.ml", line 181, characters 45-53:
181 | if List.mem "noconv" flags
^^^^^^^^
Error (warning 6 [labels-omitted]): label set was omitted in the application of this function.