Amend r228822 by not directly adding to CWARNFLAGS, but to an optional

CWARNEXTRA variable, which gets included into the initial CWARNFLAGS
setting.  This makes it easier to override CWARNFLAGS with completely
custom settings (including enabling any disabled warnings).

Reminded by:	arundel
MFC after:	1 week

sys/conf/kern.mk

@@ -6,7 +6,8 @@
 CWARNFLAGS?=	-Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual \
 		-Wundef -Wno-pointer-sign -fformat-extensions \
-		-Wmissing-include-dirs -fdiagnostics-show-option
+		-Wmissing-include-dirs -fdiagnostics-show-option \
+		${CWARNEXTRA}
 #
 # The following flags are next up for working on:
 #	-Wextra
@@ -22,7 +23,7 @@ NO_WSHIFT_COUNT_OVERFLOW=	-Wno-shift-count-overflow
 # Several other warnings which might be useful in some cases, but not severe
 # enough to error out the whole kernel build.  Display them anyway, so there is
 # some incentive to fix them eventually.
-CWARNFLAGS+=	-Wno-error-tautological-compare
+CWARNEXTRA+=	-Wno-error-tautological-compare
 .endif
 
 #