build: remove --disable-ipv6

Building with IPv6 disabled tends to break rather often and sprinkles
ugly #ifdefs around the code.  All that only to support systems where
the C library doesn't have IPv6 capability.

The year now being 2015, if this is a problem the thing to fix is the C
library.

The implication of this patch is that future patches need not care about
HAVE_IPV6 = 0 and may remove ifdefs gratuitously.  This patch doesn't
remove these ifdefs to not create unneccessary churn.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Acked-by: Paul Jakma <paul@jakma.org>
2 files changed