inttest/port2/rebar.config

{port_specs, [
    {"(darwin|linux|freebsd)", "priv/test.so",
         ["c_src/*.c"], [
            {env, [
                {"CFLAGS", "$CFLAGS -DTEST1=test1"},
                {"ERL_CFLAGS", "$ERL_CFLAGS -DTEST2=test2"}
            ]}
         ]}
]}.