On 11, there is no freebsd-version, fallback to uname.
Discovered by:
https://lists.freebsd.org/pipermail/freebsd-ports/2020-May/118597.html
sh: freebsd-version: not found make[5]: "/home/indexbuild/tindex/ports/sysutils/etc_os-release/Makefile" line 18: warning: "freebsd-version -u" returned non-zero status