GNU sort, same-source packages

Two somewhat unrelated changes (I can split the request if desired) addressing issues discussed on the list:
1. Complete adding of GNU sort by listing coreutils in the requirements section and checking that $(SORT) is runnable.
2. Add comments to remind people of updating the version of same-source packages at the same time. It won't affect "make update", of course, but the reminder will be visible in the diff at least.