The AI_NUMERICSERV constant is missing from old Darwin systems, so only
use it if it's supported.

	* include/experimental/internet [AI_NUMERICSERV]
	(resolver_base::numeric_service): Define conditionally.
	* testsuite/experimental/net/internet/resolver/base.cc: Test it
	conditionally.
	* testsuite/experimental/net/internet/resolver/ops/lookup.cc:
	Likewise.

From-SVN: r267809