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/packaging/libldns.pc.in

@@ -0,0 +1,13 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: ldns
+Description: Library for DNS programming
+URL: http://www.nlnetlabs.nl/projects/ldns
+Version: @PACKAGE_VERSION@
+Requires: 
+Libs: -L${libdir} -lldns
+Libs.private: @LDFLAGS@
+Cflags: -I${includedir}