close #1: server responds from wrong endpoint (ipv6 address)

RFC5014 (http://tools.ietf.org/html/rfc5014#section-6) support was added to
Linux back in 2008, but didn't make it to the user-level headers until
version 3.7.  If we're running on linux, and it looks like the necessaries
aren't defined, provide them.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>