OK, so asdf-unix must be turned on when darwin is present, even though
unix is absent. And/or ECL should ensure that unix is present on
darwin.
ECL currently trusts the host and uses the C compiler to decide which features it turns on. Apple's compiler does not report "unix" and hence ECL also does not. I really do not know what asdf-unix means here: is it directory convention? POSIX environment? Unix-V?