[MLton] compiler dependencies

[Stephen Weeks]

> > > Yes, there should be a dependence of MLton on gcc.
> >
> > OK.  I added it.
> 
> What about Cygwin, MinGW, and/or VC++?

"dependence" is being used in the technical Debian sense of a package
requiring another to be installed.  So, Cygwin and MinGW don't matter.

As to VC++, I've never tried it.  We don't use gcc for much when we're
using our native code generator, so it shouldn't be too hard to use
another C compiler in that case.  But I don't know why we would want
to.