Use polyc for all compilation in configure.sml

I don't think I can use polyc in the creation of a new heap that is done
within buildheap though, which means that I believe we still have to
tell users to build poly with --enable-shared.

Progress with #292

Make our copy of mllex use the 2002 Basis Library.

Remove need for separate tools-poly/mllex directory.
Also change user configuration experience slightly; put your poly
paths into a file called tools-poly/poly-includes.ML. In the form
val poly = "<path to poly executable>"
val polymllibdir = "<path to directory containing poly's lib files>"
This way we get to avoid changing tools-poly/configure.sml just to get
our own builds working.

Make the organsiation of the mllex source-code a bit cleaner so that
the polyml version can be built more easily. If you have polyml
installed nicely, it should be possible to get a poly version of mllex
by just going
make -f Holmakefile mllex.exe
in the tools/mllex directory. The tricksiness in Holmakefile is more
cute than useful, as it's not used by Holmake.