According bug #230351, xosview was broken and a fix had been committed. This PR is closed. But the port still says BROKEN_FreeBSD_12 which doesn't seem to be correct. xosview actually compiles and works fine for me. Hence I suggest to remove the BROKEN_FreeBSD_12 tag.
A commit references this bug: Author: fernape Date: Mon Dec 31 17:04:35 UTC 2018 New revision: 488803 URL: https://svnweb.freebsd.org/changeset/ports/488803 Log: sysutils/xosview: update to 1.21 No info for this new release. While here: * Remove BROKEN_* since this builds fine in 12 or greater. * Simplify patch since some changes where merged upstream: https://github.com/hills/xosview/pull/11 PR: 234526 Reported by: freebsd@oldach.net Changes: head/sysutils/xosview/Makefile head/sysutils/xosview/distinfo head/sysutils/xosview/files/patch-bsd_kernel.cc
Thanks Helge for reporting this. I updated the port to 1.21 and removed the BROKEN_* variables since this builds fine in 12.0 RELEASE and 13-CURRENT. Happy New Year!