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/python/examples/ldns-mx.py

@@ -0,0 +1,15 @@
+#!/usr/bin/python
+#
+# MX is a small program that prints out the mx records for a particular domain
+#
+import ldns
+
+resolver = ldns.ldns_resolver.new_frm_file("/etc/resolv.conf")
+
+pkt = resolver.query("nic.cz", ldns.LDNS_RR_TYPE_MX,ldns.LDNS_RR_CLASS_IN)
+
+if (pkt):
+    mx = pkt.rr_list_by_type(ldns.LDNS_RR_TYPE_MX, ldns.LDNS_SECTION_ANSWER)
+    if (mx):
+       mx.sort()
+       print mx