Use system ldns library

This library is used only by mod_enum and is available on most
platforms.  The version in our tree has many code quality problems
that are generating noise when doing static analysis.  Suffice it to
say we want rid of it as our responsibility.

FS-353

src/mod/applications/mod_enum/Makefile.am

@@ -1,24 +1,16 @@
 include $(top_srcdir)/build/modmake.rulesam
 MODNAME=mod_enum
 
-LDNS=ldns
-LDNS_DIR=$(switch_srcdir)/libs/$(LDNS)
-LDNS_BUILDDIR=$(switch_builddir)/libs/$(LDNS)
-LDNS_LA=$(LDNS_BUILDDIR)/libldns.la
-
+if HAVE_LDNS
 mod_LTLIBRARIES = mod_enum.la
 mod_enum_la_SOURCES  = mod_enum.c
-mod_enum_la_CFLAGS   = $(AM_CFLAGS) -I$(LDNS_DIR) -I$(LDNS_DIR)/include -I$(LDNS_BUILDDIR)
-mod_enum_la_LIBADD   = $(switch_builddir)/libfreeswitch.la $(LDNS_LA)
+mod_enum_la_CFLAGS   = $(AM_CFLAGS)
+mod_enum_la_CPPFLAGS = $(LDNS_CFLAGS) $(AM_CPPFLAGS)
+mod_enum_la_LIBADD   = $(LDNS_LIBS) $(switch_builddir)/libfreeswitch.la
 mod_enum_la_LDFLAGS  = -avoid-version -module -no-undefined -shared
-
-BUILT_SOURCES=$(LDNS_BUILDDIR)/ldns/util.h
-
-$(LDNS_BUILDDIR)/ldns/util.h $(LDNS_BUILDDIR)/Makefile: $(LDNS_DIR)/Makefile.in
-	mkdir -p $(LDNS_BUILDDIR)
-	cd $(LDNS_BUILDDIR) && $(DEFAULT_VARS) $(LDNS_DIR)/configure $(DEFAULT_ARGS) --srcdir=$(LDNS_DIR) --disable-gost --without-ssl --disable-sha2 --disable-shared --with-pic
-	$(TOUCH_TARGET)
-
-$(LDNS_LA): $(LDNS_BUILDDIR)/Makefile
-	cd $(LDNS_BUILDDIR) && $(MAKE)
-	$(TOUCH_TARGET)
+else
+install: error
+all: error
+error:
+	$(error You must install libdns-dev to build mod_enum)
+endif