[MLton] gmp on Darwin

[Matthew Fluet]

I got an Intel MacPro for an office machine, and, no suprise, am trying to 
build mlton.  John's binary works fine, having installed gmp via fink.

However, when I try to build from SVN sources, building the runtime fails 
because /sw/include isn't in the default include path and #include "gmp.h" 
fails.

Now, I know various ways to fix this, but I'm curious as to why it hasn't 
been an issue for anyone who has built on Darwin previously.