Fix IP_PKTINFO related compilation failure on NetBSD 7.0

Fix IP_PKTINFO related compilation failure on NetBSD 7.0

NetBSD has introduced IP_PKTINFO and struct in_pktinfo, but does not
have the "ipi_spec_dst" structure element, causing compilation errors.

Introduce a check for that (AC_CHECK_MEMBER) in configure.ac, and
change all "#ifdef HAVE_IN_PKTINFO" to also check "HAVE_IPI_SPEC_DST".

Patch inspired by NetBSD pkgsrc patch set.

(Note: with that patch, OpenVPN --multihome is still broken for IPv4
on NetBSD 7.0.1 / amd64, but that's a different issue)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20160916190242.44897-1-gert@greenie.muc.de>
URL: http://www.mail-archive.com/search?l=mid&q=20160916190242.44897-1-gert@greenie.muc.de
Signed-off-by: Gert Doering <gert@greenie.muc.de>