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/03-run.tpkg/03-run.dsc

@@ -0,0 +1,15 @@
+BaseName: 03-run
+Version: 1.0
+Description: run drill and see if it works
+CreationDate: Tue Nov  1 14:29:37 CET 2005
+Maintainer: Miek Gieben
+Category: 
+Component:
+Depends: 
+Help: 03-run.help
+Pre: 
+Post:
+Test: 03-run.test
+AuxFiles: 
+Passed:
+Failure: