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/compat/gmtime_r.c

@@ -0,0 +1,14 @@
+#ifdef HAVE_CONFIG_H
+#include <ldns/config.h>
+#endif
+
+#ifdef HAVE_TIME_H
+#include <time.h>
+#endif
+
+struct tm *gmtime_r(const time_t *timep, struct tm *result)
+{
+	/* no thread safety. */
+	*result = *gmtime(timep);
+	return result;
+}