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/masterdont/Makefile

@@ -0,0 +1,25 @@
+LDNSDIR=..
+CC=gcc
+CFLAGS=-g3 -W -Wall -Wextra -pedantic -I$(LDNSDIR)
+CFLAGS+=-D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -D__EXTENSIONS__
+LFLAGS=-L$(LDNSDIR)/lib -lcrypto
+LIBS=-lldns
+STATIC=$(LDNSDIR)/lib/libldns.a -lcrypto
+
+OBJ=main.o zones.o zinfo.o server.o process.o config_file.o
+
+all:	masterdont
+
+all-static: $(OBJ)
+	$(CC) -o masterdont $(CFLAGS) $(OBJ) $(STATIC)
+
+masterdont: $(OBJ)
+	$(CC) -o masterdont $(CFLAGS) $(OBJ) $(LFLAGS) $(LIBS)
+
+depend:
+	$(CC) $(CFLAGS) -MM *.c > makefile.dep 
+
+clean:
+	rm -f $(OBJ) masterdont
+
+-include makefile.dep