[MLton] compiler independent MLB files

[Stephen Weeks]

> 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 :-).  I guess we could
have both around (possibly pointing to the same place), the idea being
that SML_LIB is for "portable" stuff and MLTON_ROOT is for
MLton-specific stuff.  But I don't know if it's worth the trouble of
trying to express that, as only MLton and the ML Kit are the ones
supporting MLBs.  Plus as libraries get ported, things may change.

Probably better is to have a single variable (I lean toward SML_LIB).
We can keep around MLTON_ROOT for backwards compatibility.