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/MANIFEST

@@ -0,0 +1,38 @@
+Changes
+constants.PL
+dist.ini
+LDNS.xs
+lib/DNS/LDNS.pm
+lib/DNS/LDNS/DNSSecDataChain.pm
+lib/DNS/LDNS/DNSSecName.pm
+lib/DNS/LDNS/DNSSecRRs.pm
+lib/DNS/LDNS/DNSSecRRSets.pm
+lib/DNS/LDNS/DNSSecTrustTree.pm
+lib/DNS/LDNS/DNSSecZone.pm
+lib/DNS/LDNS/GC.pm
+lib/DNS/LDNS/Key.pm
+lib/DNS/LDNS/KeyList.pm
+lib/DNS/LDNS/Packet.pm
+lib/DNS/LDNS/RBNode.pm
+lib/DNS/LDNS/RBTree.pm
+lib/DNS/LDNS/RData.pm
+lib/DNS/LDNS/Resolver.pm
+lib/DNS/LDNS/RR.pm
+lib/DNS/LDNS/RRList.pm
+lib/DNS/LDNS/Zone.pm
+MANIFEST
+ppport.h
+README
+t/dnssec_datachain.t
+t/dnssec_zone.t
+t/key.t
+t/DNS-LDNS.t
+t/rdata.t
+t/resolver.t
+t/rr.t
+t/rrlist.t
+t/testdata/key.private
+t/testdata/myzone.org
+t/testdata/resolv.conf
+t/zone.t
+typemap