The smartsnmpd SConstruct file only accepts setting CFLAGS and does not
use CPPFLAGS, so pass both down using CFLAGS.

This fixes build errors with external toolchains that don't
automatically search for headers in $(STAGING_DIR).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

smartsnmpd is an implementation of SNMP Agent developed by Credo Semi. It
use Lua as script language to write SNMP MIB nodes to improve the efficiency
of developtment.

This package add native support for OpenWrt. Include using ubus and uci to
get status/info. And, it use uloop as low level event library. So it's
some sort of desgin for OpenWrt.

Website: https://github.com/credosemi/smartsnmp

Signed-off-by: Leo Ma <leoma@credosemi.com>
Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>