To complete my previous answer, I update the minor (the third number)
when new symbols are added. So the ABI changes, but no rebuild is
necessary because the old symbols are still there (with the same
signature and semantics).
However, new versions of software such as s6, including bugfix
releases, may depend on the new symbols; I document the minimum
supported version in the INSTALL and doc/upgrade.html files of these
software packages. So when you're building distro packages with
the new versions of the software, make sure to keep the dependencies
up-to-date.
--
Laurent
