Fix libdns CFLAGS

Since lib/dns/include/dns/view.h unconditionally defines dnstap-related
fields in struct dns_view (and includes <dns/dnstap.h>), care must be
taken to ensure that any source file which includes <dns/view.h> gets
built with a set of CFLAGS which allows <dns/dnstap.h> to be properly
processed (particularly its <fstrm.h> and <protobuf-c/protobuf-c.h>
conditional dependencies which are only included for dnstap-enabled
builds).  Ensure that by making LIBDNS_CFLAGS include DNSTAP_CFLAGS when
building with dnstap support.

The same reasoning applies for LMDB_CFLAGS.

Makefile.top

@@ -42,6 +42,16 @@ LIBDNS_CFLAGS = \
 LIBDNS_LIBS = \
 	$(top_builddir)/lib/dns/libdns.la
 
+if HAVE_DNSTAP
+LIBDNS_CFLAGS +=					\
+	$(DNSTAP_CFLAGS)
+endif HAVE_DNSTAP
+
+if HAVE_LMDB
+LIBDNS_CFLAGS +=					\
+	$(LMDB_CFLAGS)
+endif HAVE_LMDB
+
 LIBNS_CFLAGS = \
 	-I$(top_srcdir)/lib/ns/include