[MLton] Build fails on "unset ARCHNOPSYS ARCH OPSYS HEAP_SUFFIX"

[Matthew Fluet]

> When I tried building MLton on a university server it failed. I tracked
> down the cause to the command in the subject line. The reason is that the
> variable ARCHNOPSYS does not get defined by the .arch-n-opsys script (!)
> of the SML/NJ version installed on the server.
> 
>   $ .arch-n-opsys
>   ARCH=x86; OPSYS=linux; HEAP_SUFFIX=x86-linux
> 
>   $ sml
>   Standard ML of New Jersey v110.45 [FLINT v1.5], February 13, 2004
> 
> Below is one possible way to fix the problem (for good), but it uses sed,
> which may be undesirable. 

Thanks for the fix.

> Is it really imperative to unset the variables?

Probably not.  In fact, we probably should clean up the mlton-script to 
disallow a SML/NJ built MLton, since such a beast is really only useful 
for MLton developers.  We could add a separate mlton-nj script for 
invoking a SML/NJ built MLton.  The only complication there is that we 
would be duplicating a lot of the -*-opt flags.