[MLton] building mlton for darwin + intel

[Matthew Fluet]

> Have you checked your gmp build for intel macs?

Well, I install gmp using DarwinPorts, so I'm trusting that they 
configured things correctly.  I don't see any regression failures in 
IntInf functions (which is where MLton uses gmp).

I also used Fink at an earlier point, and their version of gmp worked 
fine as well.

I've never attempted to self-build gmp.

> Is there a compelling reason for the compiler (or mllex) to link 
> dynamically to the gmp library?

It is a debate that is raised every once in a while.  I think the 
feeling is that gmp is pretty widely available, and it is useful to pick 
up bug fixes when the gmp package is updated.

I'll try to build a statically linked package this afternoon.