thc-ipv6: Allow overriding CFLAGS

thc-ipv6 did not allow an external environment to override CFLAGS, which
would lead to our CFLAGS not being passed properly (relro,
optimizations, etc...)

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

package/network/ipv6/thc-ipv6/patches/000-cflags_override.patch

+diff -urN thc-ipv6-2.7/Makefile thc-ipv6-2.7.new/Makefile
+--- thc-ipv6-2.7/Makefile	2014-12-27 05:05:30.000000000 -0800
++++ thc-ipv6-2.7.new/Makefile	2017-02-04 20:55:51.679898101 -0800
+@@ -3,7 +3,7 @@
+ 
+ CC=gcc
+ #CFLAGS=-g
+-CFLAGS=-O2
++CFLAGS?=-O2
+ CFLAGS+=$(if $(HAVE_SSL),-D_HAVE_SSL,)
+ LDFLAGS+=-lpcap $(if $(HAVE_SSL),-lssl -lcrypto,)
+ PROGRAMS=parasite6 dos-new-ip6 detect-new-ip6 fake_router6 fake_advertise6 fake_solicitate6 fake_mld6 fake_mld26 fake_mldrouter6 flood_mldrouter6 fake_mipv6 redir6 smurf6 alive6 toobig6 rsmurf6 implementation6 implementation6d sendpees6 sendpeesmp6 randicmp6 fuzz_ip6 flood_mld6 flood_mld26 flood_router6 flood_advertise6 flood_solicitate6 trace6 exploit6 denial6 fake_dhcps6 flood_dhcpc6 fake_dns6d fragmentation6 kill_router6 fake_dnsupdate6 ndpexhaust6 detect_sniffer6 dump_router6 fake_router26 flood_router26 passive_discovery6 dnsrevenum6 inverse_lookup6 node_query6 address6 covert_send6 covert_send6d inject_alive6 firewall6 ndpexhaust26 fake_pim6 thcsyn6 redirsniff6 flood_redir6 four2six dump_dhcp6 fuzz_dhcps6 flood_rs6 fuzz_dhcpc6