Introducing the "lookup-constants runtime system" :)

[Henry Cejtin]

You want to support people using _prim to depend on new command line -D flags?
I guess I wouldn't bother.  If people want to do that kind of thing AND are
cross-compiling, then they would have to add the values when compiling the
code that has the constants in it.
Rather than having fixed names (like safe) I would say just have some fixed
prefix of the name: MLtonDef_ or something.  The symbols that start with that
would not go into the constants file, on the assumption that they will be
provided via -D flags.