This enumeration value is deprecated in /usr/include/resolv.h:

#define RES_AAONLY \
  __glibc_macro_warning ("RES_AAONLY is deprecated") 0x00000004

Since Gentoo is not using outdated glibc version it is safe to disable it.

--- a/dns.c
+++ b/dns.c
@@ -150,7 +150,6 @@
 	struct hostent *hp;
 
 	options = _res.options;
-	_res.options |= RES_AAONLY;
 	_res.options &= ~(RES_DEFNAMES | RES_DNSRCH);
 	hp = gethostbyaddr((char *)&a, sizeof(a), AF_INET);
 	_res.options = options;