After each FreeBSD release EoL and the following cleanup of legacy
code there's a bunch of complains from users about unexpected breakages:
To make it easier for users and to avoid the noise, let us introduce a
mechanism which disables building from ports for unsupported releases
with meaningful explanation. Bu default it adds IGNORE to be as explicit
as possible, but it may be overridden and be turned into a warning.
The OSVERSION ranges used here may also be used as a reference when
removing obsolete OSVERSION-based conditions.
Note that OSVERSION range may be excessive - I've included 11.x prior
to 11.0-RELEASE which I believe is not really supported. May also include
older 12.x. I'm not sure for our policies here.
Note that this also needs DragonFly support.