[MLton] Compiling mlton on MacOS

[Matthew Fluet]

> Also, the files bin/platform and build/lib/platform are identical.  Do
> you really need both?

You don't really have both.  bin/* are considered source -- i.e., they
appear in the CVS, you need it to build from source, etc.  build/* is
where the source is built -- i.e., you get it when you build from
source, an install would  cp -r build $(PREFIX), etc.

In an autoconf/configure world, the build directory might go away,
replaced by the --prefix flag, and copying binaries from their respective
directories.