This is a relatively recent regression. It is discussed on freebsd-stable@ here: https://lists.freebsd.org/pipermail/freebsd-stable/2017-January/086662.html If I had to guess, I'd say it was fallout from r312375: https://svnweb.freebsd.org/base?view=revision&revision=312375
Ouch! Sorry for that. Ed, do you have any plan to MFC r293040 or should I land a bandaid for this? (Can't commit within ~4 hours).
A commit references this bug: Author: delphij Date: Thu Jan 19 07:29:53 UTC 2017 New revision: 312403 URL: https://svnweb.freebsd.org/changeset/base/312403 Log: MFC r293040,293341,293435: Reduce libstand Makefile duplication. PR: 216251 Changes: _U stable/10/ stable/10/lib/libstand/Makefile stable/10/sys/boot/libstand32/Makefile stable/10/sys/boot/userboot/libstand/Makefile