compat: Define AI_NUMERICSERV as `0' if necessary

compat: Define AI_NUMERICSERV as `0' if necessary

AI_NUMERICSERV as a value for the `ai_flags' member
of `struct addrinfo' of header <netdb.h> has only
been available since:

  POSIX 1003.1-2008, Issue 7
  glibc 2.3.4
  Mac OS X 10.6
  etc.

Fortunately, its main purpose seems to be only
to optimize calls of `getaddrinfo', and because it
is meant to be a bit flag, it can therefore be
[relatively] safely ignored by defining it to have
the value zero.

Signed-off-by: Michael Witten <mfwitten@gmail.com>