Complete adding of GNU sort

Mention GNU Coreutils in the requirements section and check that
$(SORT) is available.
Add hints to update same-source pkgs in lockstep

Add comments to the version lines of packages binutils, gcc, gmp and
mpfr to remind people updating them of updating the version of certain
other packages as well that use the same source download.
Merge pull request #171 from uklauer/ch

GNU sort, same-source packages