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/lua/test2.lua

@@ -0,0 +1,22 @@
+-- source the lib file with the function
+dofile("rns-lib.lua")
+
+my_pkt = packet.new()
+
+my_rdf = rdf.new_frm_str(LDNS_RDF_TYPE_DNAME, "miek.nl")
+if my_rdf == nil then
+	print("failure")
+end
+rdf.print(my_rdf)
+
+my_rr = record.new_frm_str("www.miek.nl in a 192.168.1.1")
+record.print(my_rr)
+
+my_pkt = packet.new()
+packet.push_rr(my_pkt, LDNS_SECTION_ANSWER, my_rr)
+packet.push_rr(my_pkt, LDNS_SECTION_ANSWER, my_rr)
+packet.push_rr(my_pkt, LDNS_SECTION_ANSWER, my_rr)
+
+lua_record_insert(my_pkt, my_rr, 2)
+
+packet.print(my_pkt)