rebar.config.script

{_, {_, Deps}, CONFIG2} = lists:keytake(deps, 1, CONFIG).

{_, {_, [{_, RelxNameVer, Apps}|RelxOpts]}, CONFIG3} = lists:keytake(relx, 1, CONFIG2).
{_, {_, ErlOpts}, CONFIG4} = lists:keytake(erl_opts, 1, CONFIG3).

{Deps2, Apps2, ErlOpts2} =
    case os:getenv("ERL_EMETER_USE_LAGER", undefined) of
        undefined ->
            {Deps, Apps, ErlOpts};
        _ ->
            {[{lager, "3.6.2"}|Deps], [goldrush, lager|Apps], [{d, emeter_use_lager, true}|ErlOpts]}
    end.

{Deps3, Apps3, ErlOpts3} =
    case os:getenv("ERL_EMETER_USE_JIFFY", undefined) of
        undefined ->
            {[{jsone, "1.4.5"}|Deps2], [{jsone, load}|Apps2], ErlOpts2};
        _ ->
            {[{jiffy, "0.15.2"}|Deps2], [{xmerl, load}, {jiffy, load}|Apps2], [{d, emeter_use_jiffy, true}|ErlOpts2]}
    end.

[{erl_opts, ErlOpts3}
,{relx, [{release, RelxNameVer, Apps3}|RelxOpts]}
,{deps, Deps3}
|CONFIG4].