+AC_ARG_WITH([ipv6],
+ AS_HELP_STRING([--without-ipv6], [disable IPv6 support (default is autodetect)]),
+ [ac_cv_use_ipv6=$withval],
+ [ac_cv_use_ipv6=$unet_have_sockaddr_in6])
+AC_CACHE_CHECK([whether to use IPv6], [ac_cv_use_ipv6], [ac_cv_use_ipv6=no])
+if test x"$ac_cv_use_ipv6" != "xno" ; then
+ AC_DEFINE([IPV6], , [Enable IPv6 support])
+fi
+