[pkg-ntp-maintainers] Bug#507806: ntp will not build ipv6 support with new glibc

[Matt LaPlante]

Package: ntp
Version: 1:4.2.4p4+dfsg-7

The latest glibc has caused IPv6 to fail to build properly across many
packages including bind and apparently ntp.

The bind issue has been documented here:
https://bugs.launchpad.net/ubuntu/+source/bind9/+bug/249824
http://bugs.gentoo.org/show_bug.cgi?id=227333

It is now also apparent that this bug hits the ntp package as well.
The current build fails to configure IPv6 with the same symptoms:
checking for IPv6 structures... yes
checking for Kame IPv6 support... no
checking for struct if_laddrconf... no
checking for struct if_laddrreq... no
checking for struct sockaddr_in6.sin6_scope_id... yes
---> checking for in6_pktinfo... no -- disabling runtime ipv6 support
checking for sin6_scope_id in struct sockaddr_in6... yes
checking for in6addr_any... yes
checking for interface list sysctl... no


This can be rectified by adding the following to configure.ac and
regenerating the configuration file.
.............
dnl Grab any initial CFLAGS so we can pick better defaults.
iCFLAGS="$CFLAGS"
+
+dnl Define _GNU_SOURCE, this should be done before any feature test.
+AC_DEFINE(_GNU_SOURCE)

dnl check these early to avoid autoconf warnings
AC_AIX
AC_MINIX
.............

See also:
https://bugs.launchpad.net/ubuntu/+source/ntp/+bug/305043