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/t/resolver.t

@@ -0,0 +1,23 @@
+use Test::More tests => 3;
+
+use FindBin qw/$Bin/;
+
+use DNS::LDNS ':all';
+
+BEGIN { use_ok('DNS::LDNS') };
+
+my $r = new DNS::LDNS::Resolver(filename => "/etc/resolv.conf");
+
+$r->set_random(0);
+
+my $p = $r->query(
+   new DNS::LDNS::RData(LDNS_RDF_TYPE_DNAME, 'org'),
+   LDNS_RR_TYPE_SOA, LDNS_RR_CLASS_IN, LDNS_RD);
+
+isa_ok($p, 'DNS::LDNS::Packet', 'Make a simple query');
+
+my $r2 = new DNS::LDNS::Resolver(filename => "$Bin/testdata/resolv.conf");
+
+$r2->set_rtt(2, 3);
+my @rtt = $r2->rtt;
+is_deeply(\@rtt, [2, 3], "set_rtt and rtt");