Can we adopt the `MODEL:=default` behavior from [druntime](https://github.com/D-Programming-Language/druntime/blob/master/posix.mak#L38) and [phobos](https://github.com/D-Programming-Language/phobos/blob/master/posix.mak#L51).
```
MODEL:=default
ifneq (default,$(MODEL))
      MODEL_FLAG:=-m$(MODEL)
endif
```