Right now this is only handling the interpreter path, but we could do
more. I tried to add the hg rule as a dependency of everything that
might need it to work, but I didn't test all the packaging rules.
Any sane way to get this to strip $(DESTDIR) from the lead? (Or maybe just override it via another parameter.) That would solve one of the issues with the custom Mac install builder (D6846), but probably causes a problem running contrib/genosxversion.py (which wants to exec hg).