configure: Allow HARP to build with Boehm.

This isn't a documented option yet as other changes to GC build
options will be happening and we'll document it all at once.
configure: Changes to GC configuration.

We now require --with-gc=boehm|mps and --with-gc-path=/path/to/gc
rather than --with-gc or --with-mps. This makes things more consistent
and a bit easier to deal with various backends supporting either or
both GCs.