Ah, I was confused because just before that I was staring at FreeBSD
bsd.sys.mk which does use CWARNFLAGS for -W* and then looked at
-Werror in ours.
History says:
revision 1.26
date: 1998-08-25 18:57:21 +0400; author: tv; state: Exp; lines: +3 -2;
Introduce CWARNFLAGS, idea from kernel Makefiles, which goes after "-Wall
..." in the CFLAGS and can be set in directory Makefile or mk.conf.
incidentally, the reason I was looking at FreeBSD bsd.sys.mk was
precisely that I needed to disable one of the warning options but they
append them to CWARNFLAGS overriding your settings instead.