[MLton] compiler independent MLB files

[Matthew Fluet]

> > It would be great if it was easy for programmers to write compiler
> > independent MLB files.
> ...
> > One solution could be to enforce that a path variable $(MLBLIB_ROOT)
> > is set by default.
> ...
> > Any thoughts on this?
>
> Sounds great.  I wonder about the name.  Other possibilities are:
>
> 	MLB_LIB_ROOT
> 	SML_LIB_ROOT
> 	SML_ROOT
> 	LIB_ROOT
> 	SML_LIB
> 	SML
> 	LIB
> 	ROOT
>
> Initially, we called the variable SML_LIB, but somewhere along the
> way, the name got changed to MLTON_ROOT, as I recall for no great
> reason other than that we like the name MLton :-).

For a brief period of time, we were using system environment variables
for path variables, and I felt that getting MLTON into the name would be
less likely to conflict with other environment variables.  We quickly
decided that polluting the environment variable namespace was the wrong
way to go, so path variables became their own thing.