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/ldnsx/examples/ldnsx-mx2.py

@@ -0,0 +1,17 @@
+#!/usr/bin/python
+#
+# MX is a small program that prints out the mx records for a particular domain
+#
+import ldnsx
+
+resolver = ldnsx.resolver()
+
+pkt = resolver.query("nic.cz", "MX")
+if pkt:
+    for rr in pkt.answer(rr_type = "MX"):
+        rdf = rr.owner()
+        print rr
+		#Could also do:
+		#print rr[0], rr[1], rr[2], rr[3], " ".join(rr[4:])
+		#print rr.owner(), rr.ttl(), rr.rr_clas(), rr.rr_type(), " ".join(rr[4:])
+