Add files to build ruby artifact for mac on kokoro (#4814)

* Add files to build ruby artifact for mac on kokoro

* Move ruby specific stuff to ruby dir

kokoro/release/macos/build_artifacts.cfg

+# Configuration for Mac OSX release builds
+build_file: "protobuf/kokoro/release/macos/build_artifacts.sh"
+
+action {
+  define_artifacts {
+    regex: "github/protobuf/artifacts/**"
+  }
+}

kokoro/release/macos/build_artifacts.sh

+#!/bin/bash
+
+set -ex
+
+# change to repo root
+cd $(dirname $0)/../../..
+
+# all artifacts come here
+mkdir artifacts
+export ARTIFACT_DIR=$(pwd)/artifacts
+
+# ruby environment
+bash kokoro/release/macos/ruby/ruby_build_environment.sh
+
+gem install rubygems-update
+update_rubygems
+
+# build artifacts
+bash kokoro/release/macos/ruby/ruby_build.sh

kokoro/release/macos/ruby/ruby_build.sh

+#!/bin/bash
+
+# Build protoc
+if test ! -e src/protoc; then
+  ./autogen.sh
+  ./configure
+  make -j4
+fi
+
+umask 0022
+pushd ruby
+bundle install && bundle exec rake gem:native
+ls pkg
+mv pkg/* $ARTIFACT_DIR
+popd

kokoro/release/macos/ruby/ruby_build_environment.sh

+#!/bin/bash
+
+set -ex
+
+set +ex  # rvm script is very verbose and exits with errorcode
+source $HOME/.rvm/scripts/rvm
+set -e  # rvm commands are very verbose
+time rvm install 2.5.0
+rvm use 2.5.0 --default
+gem install bundler --no-ri --no-doc
+rvm osx-ssl-certs status all
+rvm osx-ssl-certs update all
+set -ex
+
+rm -rf ~/.rake-compiler
+
+CROSS_RUBY=$(mktemp tmpfile.XXXXXXXX)
+
+curl https://raw.githubusercontent.com/rake-compiler/rake-compiler/v1.0.3/tasks/bin/cross-ruby.rake > "$CROSS_RUBY"
+
+# See https://github.com/grpc/grpc/issues/12161 for verconf.h patch details
+patch "$CROSS_RUBY" << EOF
+--- cross-ruby.rake	2018-04-10 11:32:16.000000000 -0700
++++ patched	2018-04-10 11:40:25.000000000 -0700
+@@ -133,8 +133,10 @@
+     "--host=#{MINGW_HOST}",
+     "--target=#{MINGW_TARGET}",
+     "--build=#{RUBY_BUILD}",
+-    '--enable-shared',
++    '--enable-static',
++    '--disable-shared',
+     '--disable-install-doc',
++    '--without-gmp',
+     '--with-ext='
+   ]
+ 
+@@ -151,6 +153,7 @@
+ # make
+ file "#{USER_HOME}/builds/#{MINGW_HOST}/#{RUBY_CC_VERSION}/ruby.exe" => ["#{USER_HOME}/builds/#{MINGW_HOST}/#{RUBY_CC_VERSION}/Makefile"] do |t|
+   chdir File.dirname(t.prerequisites.first) do
++    sh "test -s verconf.h || rm -f verconf.h"  # if verconf.h has size 0, make sure it gets re-built by make
+     sh MAKE
+   end
+ end
+EOF
+
+MAKE="make -j8"
+
+for v in 2.5.0 2.4.0 2.3.0 2.2.2 2.1.6 2.0.0-p645 ; do
+  ccache -c
+  rake -f "$CROSS_RUBY" cross-ruby VERSION="$v" HOST=x86_64-darwin11 MAKE="$MAKE"
+done
+
+sed 's/x86_64-darwin-11/universal-darwin/' ~/.rake-compiler/config.yml > "$CROSS_RUBY"
+mv "$CROSS_RUBY" ~/.rake-compiler/config.yml
+