Merge branch '836-building-fails-in-build-subdirectory-when-dnstap-is-enabled' into 'master'

Resolve "Building fails in build subdirectory when dnstap is enabled"

Closes #836

See merge request !1510

CHANGES

+5163.	[cleanup]	Out-of-tree builds failed --enable-dnstap. [GL #836]
+
 5162.	[cleanup]	Improve dnssec-keymgr manual. Thanks to Tony Finch.
 			[GL !1518]
 

lib/dns/Makefile.in

@@ -205,7 +205,7 @@ ${OBJS}: include
 dnstap.@O@: dnstap.c dnstap.pb-c.c
 
 dnstap.pb-c.c dnstap.pb-c.h include/dns/dnstap.pb-c.h: dnstap.proto
-	$(PROTOC_C) --c_out=. dnstap.proto
+	$(PROTOC_C) --c_out=. --proto_path ${srcdir} dnstap.proto
 	cp -f dnstap.pb-c.h include/dns
 
 dnstap.pb-c.@O@: dnstap.pb-c.c