[MLton] Compiling mlton on MacOS

[Stephen Weeks]

> I made some progress on the port, before hitting an MLRISC bug in
> SML/NJ (I hope to get that fixed for 110.45).

OK.  If waiting on 110.45 slows development too much, we could try
proceeding by cross development from a Linux machine.  Cross
development is getting easier these days.  I do it without even having
a cross gcc - I work by generating the C on the Linux machine,
shipping it to the target machine, and running there.  Also, since
MLton now has a decent front end, it is easier to work without SML/NJ
entirely.

> Do you want my diffs so far, or do you want to wait until I get a
> working version?

Go ahead and send them.  There are other MacOS developers who might
want to have a look.

> There are a couple of things that I think could be improved in the
> porting process.  One is that you should move to testing features
> instead of operating system in your header files.

That makes sense.  I'll look at your patch to see where this might be
done.

> In the long run, I think that you should consider using autoconf.

We'll look into this.