rebar.config.script

{Tgt,Ext} = case os:type() of
              {win32, _}   -> {".win", ".dll"};
              {unix,linux} -> {".linux",".so"};
              _            -> {"", ""}
            end,
Arch      = case erlang:system_info(wordsize) of
              4 -> ".x86";
              8 -> ".x64"
            end,

Driver    = "priv/sqlite3_drv" ++ Tgt ++ Arch ++ Ext,

case {filelib:is_regular(Driver), os:getenv("REBUILD")} of
  {true, false} ->
    io:format("No need to rebuild '~s'\n", [Driver]),
    {port_specs,[{V,_}|_]} = lists:keyfind(port_specs, 1, CONFIG),
    Cfg1     = lists:keyreplace(port_specs, 1, CONFIG, {port_specs, [{V, []}]}),
    [{artifacts, [Driver]} | Cfg1];
  _ ->
    PortSpec = {port_specs, [{Driver, ["c_src/*.c", "sqlite3_amalgamation/sqlite3.c"]}]},
    PortEnv  = proplists:get_value(port_env, CONFIG),
    PortEnv1 = [{"CFLAGS", "-DDRIVER_SFX=\\\""++Tgt++Arch++"\\\""} | PortEnv],
    Cfg1     = lists:keyreplace(port_env,   1, CONFIG, {port_env, PortEnv1}),
    Cfg2     = lists:keyreplace(port_specs, 1, Cfg1,   PortSpec),
    [{artifacts, [Driver]} | Cfg2]
end.