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>
(cherry picked from commit 9b2321f42d075a12e73a81b89bd49b8696d3700f)
