fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/contrib/DNS-LDNS/lib/DNS/LDNS/RBTree.pm

@@ -0,0 +1,59 @@
+package DNS::LDNS::RBTree;
+
+use 5.008008;
+use strict;
+use warnings;
+
+use DNS::LDNS;
+
+our $VERSION = '0.61';
+
+# Note: Since this class does not have a constructor, we can let its child
+# objects be owned by the parent. This reduces the recursion depth on
+# DESTROY.
+
+sub first {
+    my $self = shift;
+    return DNS::LDNS::GC::own($self->_first, $self);
+}
+
+sub last {
+    my $self = shift;
+    return DNS::LDNS::GC::own($self->_last, $self);
+}
+
+sub DESTROY {
+    DNS::LDNS::GC::free($_[0]);
+}
+
+1;
+__END__
+
+=head1 NAME
+
+DNS::LDNS::RBTree - Tree of DNSSecName nodes
+
+=head1 SYNOPSIS
+
+  use DNS::LDNS ':all'
+
+  rbnode = rbtree->first
+  rbnode = rbtree->last
+
+=head1 SEE ALSO
+
+http://www.nlnetlabs.nl/projects/ldns
+
+=head1 AUTHOR
+
+Erik Pihl Ostlyngen, E<lt>erik.ostlyngen@uninett.noE<gt>
+
+=head1 COPYRIGHT AND LICENSE
+
+Copyright (C) 2013 by UNINETT Norid AS
+
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself, either Perl version 5.14.2 or,
+at your option, any later version of Perl 5 you may have available.
+
+=cut