Compile with -fno-strict-overflow

This causes signed integer & pointer overflow to have a defined
behaviour of wrapping according to two's compliment. I.e INT_MAX will
wrap to INT_MIN and vice versa.

This is mainly to cover existing cases, not an invitation to add more.

Cc: Dan Callahan <d.callahan@f5.com>
Suggested-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

auto/cc/test

@@ -65,6 +65,8 @@ case $NXT_CC_NAME in
         # Do not export symbols except explicitly marked with NXT_EXPORT.
         NXT_CFLAGS="$NXT_CFLAGS -fvisibility=hidden"
 
+        NXT_CFLAGS="$NXT_CFLAGS -fno-strict-overflow"
+
         # c99/gnu99 conflict with Solaris XOPEN.
         #NXT_CFLAGS="$NXT_CFLAGS -std=gnu99"
 
@@ -107,6 +109,8 @@ case $NXT_CC_NAME in
         # Do not export symbols except explicitly marked with NXT_EXPORT.
         NXT_CFLAGS="$NXT_CFLAGS -fvisibility=hidden"
 
+        NXT_CFLAGS="$NXT_CFLAGS -fno-strict-overflow"
+
         NXT_CFLAGS="$NXT_CFLAGS -O"
         #NXT_CFLAGS="$NXT_CFLAGS -O0"
         NXT_CFLAGS="$NXT_CFLAGS -Wall -Wextra"