[MLton] problem building from svn checkout

[Matthew Fluet]

> The only way I see that the hack in front-end/sources.mlb failing is
> if SML_LIB is pointing in some strange place.  But, I would expect
> that to show up as an error in the MLB not being able to find
> unsafe.mlb or something like that.  Further, your error happened on
> the second round of compilation, so it was using the just-built MLton,
> which should have all the build/lib stuff in place, in particular
> mlb-path-map.  So, I am stumped.  Perhaps you could investigate a bit
> more where unsafe.mlb is coming from and why it isn't making Unsafe
> available.

One thing you can try is

 cd mlton
 ../build/bin/mlton -stop f mlton.mlb

which will show all the files reachable from mlton.mlb.  You can search 
that list for unsafe.sml and unsafe.sig.