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.