[MLton] Two small patches

[Stephen Weeks]

> Here's one patch to add a pair of easy functions to mingw.  The
> added cast makes explicit the return cast which happens (and is
> correct).

Checked in.  Thanks.

BTW, when enabling such functions, sometimes one also needs to edit

	basis-library/posix/stub-mingw.sml

> I also wonder whether permission changing methods should silently
> succeed?  Owner checking and getuid, etc might also sensibly return
> the same constant.  That's what cygwin does under at least Win95.

I think it's better to raise SysErr than to silently return.  That way
a programmer is warned about the unusual behavior.  It's easy enough
for a programmer to wrap the Posix structure if they want.

> Another patch moves the CFLAGS from CC as is common convention.

Checked in.