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/16-unit-tests-edns.tpkg/16-unit-tests-edns.test

@@ -0,0 +1,14 @@
+# #-- 16-unit-tests-edns.test --#
+# source the master var file when it's there
+[ -f ../.tpkg.var.master ] && source ../.tpkg.var.master
+# use .tpkg.var.test for in test variable passing
+[ -f .tpkg.var.test ] && source .tpkg.var.test
+# svnserve resets the path, you may need to adjust it, like this:
+#PATH=$PATH:/usr/sbin:/sbin:/usr/local/bin:/usr/local/sbin:.
+
+export LD_LIBRARY_PATH="../../lib:$LD_LIBRARY_PATH"
+export DYLD_LIBRARY_PATH="../../lib:$DYLD_LIBRARY_PATH"
+
+# run the test
+./16-unit-tests-edns
+exit $?