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/25-ZONEMD.tpkg/A.1.Simple-EXAMPLE-Zone

@@ -0,0 +1,13 @@
+example.      86400   IN  SOA     ns1 admin 2018031900 (
+                                  1800 900 604800 86400 )
+              86400   IN  NS      ns1
+              86400   IN  NS      ns2
+              86400   IN  ZONEMD  2018031900 1 1 (
+                                  c68090d90a7aed71
+                                  6bc459f9340e3d7c
+                                  1370d4d24b7e2fc3
+                                  a1ddc0b9a87153b9
+                                  a9713b3c9ae5cc27
+                                  777f98b8e730044c )
+ns1           3600    IN  A       203.0.113.63
+ns2           3600    IN  AAAA    2001:db8::63