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-mx1.py

@@ -0,0 +1,11 @@
+import ldnsx
+
+resolver = ldnsx.resolver()
+
+pkt = resolver.query("nic.cz", "MX")
+
+if (pkt):
+    mx = pkt.answer()
+    if (mx):
+       mx.sort()
+       print mx