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/28-tab-instead-of-space.tpkg/28-tab-instead-of-space.db

@@ -0,0 +1 @@
+com. 14400 IN NAPTR 100   10 S SIP+D2U "!^.*$!sip:customer-service@example.com!"	com.

zonemaster-ldns/ldns/test/28-tab-instead-of-space.tpkg/28-tab-instead-of-space.dsc

@@ -0,0 +1,16 @@
+BaseName: 28-tab-instead-of-space
+Version: 1.0
+Description: Check for GH 157 regression
+CreationDate: Tue Mar 15 21:01:34 EST 2022
+Maintainer: 
+Category: 
+Component:
+CmdDepends: 
+Depends: 
+Help: 28-tab-instead-of-space.help
+Pre: 
+Post: 
+Test: 28-tab-instead-of-space.test
+AuxFiles: 
+Passed:
+Failure:

zonemaster-ldns/ldns/test/28-tab-instead-of-space.tpkg/28-tab-instead-of-space.help

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

zonemaster-ldns/ldns/test/28-tab-instead-of-space.tpkg/28-tab-instead-of-space.test

@@ -0,0 +1,15 @@
+# #-- 28-tab-instead-of-space.test --#
+# source the master var file when it's there
+[ -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 28-tab-instead-of-space.db`
+
+if [[ $? -eq 0 ]]; then
+    echo $OUTPUT | grep customer-service
+else
+        exit 1
+fi