[MLton] add-cross no longer uses a cross compiler

[Stephen Weeks]

> > To be clear, I have not changed (and am not proposing to change) how
> > people use MLton for cross development.  They can still use a cross
> > gcc if they want.  What I have changed is how people build MLton for
> > new targets.
> 
> Isn't the add-cross script also to enable cross-compilation in the
> first place, since it builds the MLton runtime for cross-compilation?
> If you have these prebuilt on your website there is no need for
> people to install the native Windows gcc (for example) to do
> cross-compilation. But if people are installing from sources
> for cross-compilation they would need a native gcc first before
> they can do cross-compilation. Or did I miss something?

No.  You're right.  I was confused.  add-cross does have to be used by
those enabling cross compilation.  But once the cross runtime library
is built, they can use a gcc cross compiler if they want.

> > I have eliminated the the requirement for a cross gcc
> > and added a requirement for a native gcc on the target.  Overall, I
> > think this is a win due to the complexities of cross gcc's.
> 
> If you have the setup - I believe I agree.
> 
> But I think I still want to take the offer of a switch on
> the add-cross script - this will be the super-solution with
> all options available ;)

Sounds good.