Disallow optimizations which potentially remove boundary checks

for signed values due to a compiler authors considering integer
overflow as impossible.

The change follows suit of other projects taking the same measure.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/conf/kern.mk

@@ -147,6 +147,12 @@ INLINE_LIMIT?=	8000
 #
 CFLAGS+=	-ffreestanding
 
+#
+# Do not allow a compiler to optimize out overflow checks for signed
+# types.
+#
+CFLAGS+=	-fno-strict-overflow
+
 #
 # GCC SSP support
 #