[MLton] mlb support

[Henry Cejtin]

I have to agree with Matthew that having the equivalent of
	-show-basis <file>
be in the .mlb files seems a bit strange to me.  It isn't just that it will
cause me to change the .mlb files more frequently (to add and remove this
line) but the fact that some how it seems strange that the .mlb file is
going to control what files get written.  Of course it does this any way
for the name of the executable, but for these other things it seems more
surprising.
I would vote for the command line for -show-basis.