Pr 1770 (#1771)

* make pb

* Makefile for pb

Move the Makefile into the pb directory to remove some clutter from the
main Makefile.

Makefile

@@ -78,6 +78,10 @@ core/zplugin.go core/dnsserver/zdirectives.go: plugin.cfg
 gen:
 	go generate coredns.go
 
+.PHONY: pb
+pb:
+	$(MAKE) -C pb
+
 .PHONY: linter
 linter:
 	go get -u github.com/alecthomas/gometalinter

pb/Makefile

+# Generate the Go files from the dns.proto protobuf, you need the utilities
+# from: https://github.com/golang/protobuf to make this work.
+# The generate dns.pb.go is checked into git, so for normal builds we don't need
+# to run this generation step.
+
+all: dns.pb.go
+
+dns.pb.go: dns.proto
+	protoc --go_out=plugins=grpc:. dns.proto && \
+        sed -e s?golang.org/x/net/context?context? < dns.pb.go > dns.pb.go.tmp && \
+        mv dns.pb.go.tmp dns.pb.go