[MLton] Temporary directories on MinGW / mlnlffigen

[Matthew Fluet]

> Anyway, the cause of the problem is that MLton's libraries try to create
> temporary files into a directory named "/tmp".  This was noted earlier by
> David Hansel (http://mlton.org/pipermail/mlton/2007-March/029675.html).
> 
> Below is a patch that we're currently using to fix the problem with 
> mlnlffigen.
> While preparing the patch, I also grepped for "/tmp/" and noticed that 
> MLton's
> sources contain several other references to the directory.

The patch seems great.  I wonder if we should check for the existence of 
the temporary directory, but that may not be required.
-Matthew