[MLton] Cygwin->Mingw32: patch + future

[Matthew Fluet]

> There were only two issues:
> 1. With gcc-3.4, the '-b <arch>' must be the first argument.
> 2. When using cygwin to compile native windows applications via their
> included mingw compiler, the option to use is '-mno-cygwin' instead.
> (I don't know why they did this, but there you have it)
>
> In general,  I think most of the CC-controlling options should be moved
> into a config file somewhere. Some people might want to use 'icc' for
> example, or under windows one of those other non-free compilers. Also, the
> gcc command-line api is always changing (and recompiling MLton is brutal!).
> I'm not planning on doing this.

The CC-controling options are essentially in their own place:
in the bin/mlton script you can see the default settings for
  -cc             -- the C compiler
  -cc-opt         -- options to the C compiler
  -target-cc-opt  -- options to the C compiler on a particular target

Modifying those would be better than modifying main.fun, for per target
specifics.