Cross-compiling support.

Applied alex-eri's patch openwrt/patches/310-Makefile_crosscompile.patch
to compile for OpenWrt. Pull #61.

Author: aabc

Makefile.in

@@ -64,10 +64,10 @@ sinstall: | snmp_NETFLOW.so IPT-NETFLOW-MIB.my
 	fi
 
 %_sh.o: libipt_NETFLOW.c
-	gcc -O2 -Wall -Wunused $(IPTABLES_CFLAGS) -fPIC -o $@ -c libipt_NETFLOW.c
+	$(CC) $(CFLAGS) -O2 -Wall -Wunused $(IPTABLES_CFLAGS) -fPIC -o $@ -c libipt_NETFLOW.c
 
 %.so: %_sh.o
-	gcc -shared -o $@ $<
+	$(CC) -shared -o $@ $<
 
 version.h: ipt_NETFLOW.c ipt_NETFLOW.h compat.h Makefile
 	@./version.sh --define > version.h