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/test/tpkg/examples/test3.tpkg/test3.dsc

@@ -0,0 +1,15 @@
+BaseName: test3
+Version: 1.0 
+Description: Test negative SOA TTL, added to test BUG #6
+CreationDate: Thu Oct 27 11:57:07 CEST 2005
+Maintainer: Miek Gieben
+Category: 
+Component:
+Depends: 
+Help:
+Pre: test3.pre
+Post: test3.post
+Test: test3.test
+AuxFiles: test3.querygood
+Passed: "matched"
+Failure: