rebar.config.script

Def0 = case erlang:is_builtin(erlang, binary_to_integer, 1) andalso
            erlang:is_builtin(erlang, binary_to_float, 1) of
           true -> [];
           false -> [{d, no_binary_to_whatever}]
       end,
Def1 = case erlang:is_builtin(erlang, is_map, 1) of
           true -> [{d, maps_support}|Def0];
           false -> Def0
       end,
Defs = case os:getenv("JSX_FORCE_MAPS") of
           false -> Def1;
           _ -> [{d, maps_always}|Def1]
       end,
lists:keystore(erl_opts, 1, CONFIG,
               {erl_opts, proplists:get_value(erl_opts, CONFIG, []) ++ Defs}).