sys/capability.h will be removed before FreeBSD 13. The (now) correct header to use on all supported (and most unsupported) versions of FreeBSD is sys/capsicum.h.
There's no need for autoconf checks etc. to check for both (in the ports tree) but upstream may want to if there's a desire to build on older releases.
Maintainer informed via mail
It appears this is simply a side-effect of go's use of capability.h - see PR 233092.
grep of the source-code for "capsicum.h" shows
And no output for "capability.h".
So I close here, with overcome by events.