dist/IO: Allow to be dual-lived

This dual-lived module has not been able to be compiled on releases
earlier than 5.10.1 since, I believe, that release, and not outside of
blead since commit 6f2d5cbc in the 5.19 series, both due to using macros
that were not backported.

This commit suitably defines the current missing macro when it isn't
available.