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/26-loc-centimetre.tpkg/26-loc-centimetre.dsc

@@ -0,0 +1,16 @@
+BaseName: 26-loc-centimetre
+Version: 1.0
+Description: Verify parsing of centimetres in LOC records.
+CreationDate: di  7 sep 2021 10:58:42 CEST
+Maintainer: 
+Category: 
+Component:
+CmdDepends: 
+Depends: 
+Help: 26-loc-centimetre.help
+Pre: 
+Post: 
+Test: 26-loc-centimetre.test
+AuxFiles: 
+Passed:
+Failure: