Andrej would prefer to avoid commercial-license code in IMP, and it
would surely be a pain to remove this code in future if it became
incompatible with our license. But he prefers more to save time now, so
he's OK with linking against something like CGAL. So I can commit this
patch in essentially its current form, provided that it applies against
current SVN (it doesn't right now, e.g. because it's missing some files).