[MLton] Port to HP-UX

[Matthew Fluet]

> A patch is attached.

No patch attached.

> - I had to edit runtime/Posix/ProcEnv/Uname.c in order not to conflict
>  with an extern declaration of `utsname' in some system header.

That should be fine.

> - I edited runtime/gc.c to avoid some compiler warnings about
>  inapproriate format strings for pointer differences.

Does HP-UX use its own C compiler rather than gcc?  We don't observe 
format string warnings with gcc.

> The mlton.share test also fails.  It's a bit more mystical to me.  It
> reports bigger sizes than what the test expects.  Any suggestions on
> how should I start debugging this?

First, be certain you are using the latest SVN sources, as we recently 
fixed a bug with share.  Next, compile the regression test with
-debug true, which will check a number of assertions in the runtime.