Add protoc script which downloads and runs protoc (linkerd#1138)

* Add protoc script which downloads and runs protoc

.gitignore

@@ -7,6 +7,7 @@ project/plugins/src_managed/
 http.pid
 npm-debug.log
 .sbt-launch.jar
+.protoc
 .idea
 .iml
 .DS_Store

project/Grpc.scala

@@ -76,6 +76,7 @@ object Grpc extends Base {
       javaSource <<= (javaSource in Compile),
       scalaSource <<= (sourceManaged in Compile) { _ / "compiled_protobuf" },
       generatedTargets <<= scalaSource { d => Seq(d -> "*.pb.scala") },
+      protoc := "./protoc",
       grpcGenExec <<= grpcGenExec0,
       runProtoc <<= runProtoc0,
       protocOptions <<= scalaSource { ss =>

protoc

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+set -e
+
+if [ "$(uname -s)" = "Darwin" ]; then
+  os=osx
+else
+  os=linux
+fi
+arch=$(uname -m)
+
+protocbin=.protoc
+protocversion=3.0.0
+protocurl="https://github.com/google/protobuf/releases/download/v${protocversion}/protoc-${protocversion}-${os}-${arch}.zip"
+
+if [ ! -f "$protocbin" ]; then
+  echo "downloading $protocbin" >&2
+  tmp=$(mktemp -d -t protoc.XXX)
+  curl -L --silent --fail -o "$tmp/$protocbin.zip" "$protocurl"
+  unzip -q "$tmp/$protocbin.zip" -d "$tmp"
+  mv "$tmp/bin/protoc" "$protocbin"
+  rm -rf "$tmp"
+fi
+
+./$protocbin "$@"