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.db

@@ -0,0 +1 @@
+one 12345 IN LOC 12 45 52.333 N 105 40 33.452 W -24m 0.1m 0.1m 0.1m

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:

zonemaster-ldns/ldns/test/26-loc-centimetre.tpkg/26-loc-centimetre.help

@@ -0,0 +1 @@
+This requires no arguments.

zonemaster-ldns/ldns/test/26-loc-centimetre.tpkg/26-loc-centimetre.test

@@ -0,0 +1,13 @@
+[ -f ../.tpkg.var.master ] && source ../.tpkg.var.master
+
+export PATH=$PATH:/usr/sbin:/sbin:/usr/local/bin:/usr/local/sbin:.
+
+export LD_LIBRARY_PATH=../../lib:$LD_LIBRARY_PATH
+
+OUTPUT=`../../examples/ldns-read-zone 26-loc-centimetre.db`
+
+if [[ $? -eq 0 ]]; then 
+    echo $OUTPUT | grep 0.10m
+else
+        exit 1
+fi