Index: sys/conf/kern.mk
===================================================================
--- sys/conf/kern.mk
+++ sys/conf/kern.mk
@@ -67,8 +67,7 @@
 		-Wno-error=overflow				\
 		-Wno-error=sequence-point			\
 		-Wno-error=shift-overflow			\
-		-Wno-error=tautological-compare			\
-		-Wno-unused-but-set-variable
+		-Wno-error=tautological-compare
 .if ${COMPILER_VERSION} >= 70100
 CWARNEXTRA+=	-Wno-error=stringop-overflow
 .endif
@@ -83,6 +82,12 @@
 		-Wno-error=alloca-larger-than=
 .endif
 
+.if ${MK_SET_BUT_NOTUSED_KERNEL_WARNINGS} == "no"
+CWARNEXTRA+=	${NO_WUNUSED_BUT_SET_VARIABLE}
+.else
+CWARNEXTRA+=	-Wno-error=unused-but-set-variable
+.endif
+
 # GCC produces false positives for functions that switch on an
 # enum (GCC bug 87950)
 CWARNFLAGS+=	-Wno-return-type