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/timegm.c

@@ -0,0 +1,31 @@
+#ifdef HAVE_CONFIG_H
+#include <ldns/config.h>
+#endif
+
+#include <stdio.h>
+
+#ifdef HAVE_STDLIB_H
+#include <stdlib.h>
+#endif
+
+#include <time.h>
+
+time_t
+timegm (struct tm *tm) {
+	time_t ret;
+	char *tz;
+	
+	tz = getenv("TZ");
+	putenv((char*)"TZ=");
+	tzset();
+	ret = mktime(tm);
+	if (tz) {
+		char buf[256];
+		snprintf(buf, sizeof(buf), "TZ=%s", tz);
+		putenv(tz);
+	}
+	else
+		putenv((char*)"TZ");
+	tzset();
+	return ret;
+}