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/dnssec_zone.t

@@ -0,0 +1,35 @@
+use Test::More tests => 7;
+
+use FindBin qw/$Bin/;
+
+use DNS::LDNS ':all';
+
+BEGIN { use_ok('DNS::LDNS') };
+
+# Create a new dnssec zone
+my $z = new DNS::LDNS::DNSSecZone;
+isa_ok($z, 'DNS::LDNS::DNSSecZone', 'Create an empty zone');
+
+# Read a zone from file and create a dnssec zone from it
+my $z2 = new DNS::LDNS::Zone(
+    filename => "$Bin/testdata/myzone.org");
+
+$z->create_from_zone($z2);
+
+my $rrset = $z->find_rrset(
+    new DNS::LDNS::RData(LDNS_RDF_TYPE_DNAME, 'ns1.myzone.org.'), 
+        LDNS_RR_TYPE_A);
+
+is($rrset->rrs->rr->type, LDNS_RR_TYPE_A, 'Found an A record');
+is($rrset->rrs->rr->dname, 'ns1.myzone.org.', 'Dname is ns1.myzone.org.');
+
+is($z->add_empty_nonterminals, LDNS_STATUS_OK, 'Add empty non-terminals');
+
+my $klist = new DNS::LDNS::KeyList;
+$klist->push(new DNS::LDNS::Key(filename => "$Bin/testdata/key.private"));
+$klist->key(0)->set_pubkey_owner(
+    new DNS::LDNS::RData(LDNS_RDF_TYPE_DNAME, 'myzone.org'));
+
+is($z->sign($klist, LDNS_SIGNATURE_REMOVE_ADD_NEW, 0), LDNS_STATUS_OK, 'Sign');
+is($z->sign_nsec3($klist, LDNS_SIGNATURE_REMOVE_ADD_NEW, 1, 0, 10, 'ABBA', 0),
+   LDNS_STATUS_OK, 'Sign nsec3');