The rule is, if you change scan.l or gram.y, you bump the config(5)
version. If you implement the changes under sys/conf/files or affiliate,
you bump the required version in sys/conf/files or in an appropriate place
to minimise annoyance. If the changes makes new config(1) incompatible
with a previous version of config(5), embed it in config(1) using the
CONFIG_MINVERSION definition along with CONFIG_VERSION.
This has been in the tree for what, 3 years now? It's even documented...
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