building out-of-tree fails with dnstap enabled

Summary

Building BIND 9.11.11 and 9.14.6 out-of-tree fails with dnstap enabled. It fails to compile dnstap.proto.

BIND version used

BIND 9.11.11 (Extended Support Version) <id:4ae9ff1>
running on Linux x86_64 4.9.0-9-amd64 #1 SMP Debian 4.9.168-1+deb9u5 (2019-08-11)
built by make with '--enable-dnstap' '--without-python'
compiled by GCC 6.3.0 20170516
compiled with OpenSSL version: OpenSSL 1.1.0k  28 May 2019
linked to OpenSSL version: OpenSSL 1.1.0k  28 May 2019
compiled with libxml2 version: 2.9.4
linked to libxml2 version: 20904
compiled with zlib version: 1.2.8
linked to zlib version: 1.2.8
threads support is enabled

BIND 9.14.6 (Stable Release) <id:efd3496>
running on Linux x86_64 4.9.0-9-amd64 #1 SMP Debian 4.9.168-1+deb9u5 (2019-08-11)
built by make with '--enable-dnstap' '--without-python' '--disable-linux-caps'
compiled by GCC 6.3.0 20170516
compiled with OpenSSL version: OpenSSL 1.1.0k  28 May 2019
linked to OpenSSL version: OpenSSL 1.1.0k  28 May 2019
compiled with libxml2 version: 2.9.4
linked to libxml2 version: 20904
compiled with zlib version: 1.2.8
linked to zlib version: 1.2.8
threads support is enabled

default paths:
  named configuration:  /etc/named.conf
  rndc configuration:   /etc/rndc.conf
  DNSSEC root key:      /etc/bind.keys
  nsupdate session key: /var/run/named/session.key
  named PID file:       /var/run/named/named.pid
  named lock file:      /var/run/named/named.lock

Steps to reproduce

1. download tarball and build out-of-tree

$ tar -xzf bind-9.14.6.tar.gz
$ mkdir bind-9.14.6.obj
$ cd bind-9.14.6.obj
$ ../bind-9.14.6/configure --enable-dnstap
$ make

What is the current bug behavior?

It fails to compile dnstap.proto.

/usr/bin/protoc-c --c_out=. --proto_path ../../../bind-9.14.6/lib/dns dnstap.proto
dnstap.proto: No such file or directory

What is the expected correct behavior?

It should finish building successfully.

Relevant configuration files

(Not applicable)

Relevant logs and/or screenshots

(See above)

Possible fixes

--proto_path option is only applied to imports: not effective as proto file path specified in the argument. The proto file path should be specified explicitly.

--- lib/dns/Makefile.in.orig	2019-09-09 23:51:54.000000000 +0900
+++ lib/dns/Makefile.in	2019-09-21 01:35:29.793993190 +0900
@@ -211,7 +211,7 @@
 dnstap.@O@: dnstap.c dnstap.pb-c.c
 
 dnstap.pb-c.c dnstap.pb-c.h: dnstap.proto
-	$(PROTOC_C) --c_out=. --proto_path ${srcdir} dnstap.proto
+	$(PROTOC_C) --c_out=. --proto_path ${srcdir} ${srcdir}/dnstap.proto
 
 dnstap.pb-c.@O@: dnstap.pb-c.c