Create new dialect knob, as setting the language dialect isn't a warning flag.

sys/conf/kern.mk

@@ -12,7 +12,7 @@ CWARNFLAGS=
 .else
 CWARNFLAGS?=	-Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual \
-		${_wundef} -fformat-extensions -std=c99
+		${_wundef} -fformat-extensions
 .if !defined(NO_UNDEF)
 _wundef=	-Wundef
 .endif

sys/conf/kern.pre.mk

@@ -43,6 +43,7 @@ COPTFLAGS+= ${_CPUCFLAGS}
 .if ${CC} == "icc"
 NOSTDINC= -X
 .else
+C_DIALECT= -std=c99
 NOSTDINC= -nostdinc
 .endif
 
@@ -80,7 +81,7 @@ INCLUDES+= -I$S/gnu/fs/xfs/FreeBSD -I$S/gnu/fs/xfs/FreeBSD/support -I$S/gnu/fs/x
 
 .endif
 
-CFLAGS=	${COPTFLAGS} ${CWARNFLAGS} ${DEBUG}
+CFLAGS=	${COPTFLAGS} ${C_DIALECT} ${DEBUG} ${CWARNFLAGS}
 CFLAGS+= ${INCLUDES} -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h
 .if ${CC} != "icc"
 CFLAGS+= -fno-common -finline-limit=${INLINE_LIMIT}