[MLton] Windows ports and paths

[Matthew Fluet]

> > So, I tried rebuilding cygwin today and found that the C99 support there is
> > a bit broken. For whatever reason, several functions and structures get
> > turned off when compiled with -std=c99. An easy fix is to use -std=gnu99
> > instead. That's still C99, but with the GNU functions also.
> >
> > A patch, fix-cygwin.patch is attached (it also fixes a buggy mingw setenv).
> 
> Thanks for the patch.  I committed all the c99-->gnu99 changes.  I
> didn't commit the change from "-mcpu=pentiumpro" to
> "-march=pentiumpro", because I'm not sure what our position is on
> generating specialized code that doesn't run on some archs.  Anyone
> else have any thoughts?

I agree that we shouldn't default to generating specialized code.  
However, I think the reality is that most users will be compiling programs 
to run on their own machines (I know there are some notable exceptions).  
With MLBs, we've given users the option of having a .mlton directory, so 
maybe we out to have a simple way for users to default their compile 
options, either with a MLTON_OPTS environment variable or with a 
.mlton/options file.  That would save Wesley from needing to hack the 
mlton script each time around.