Produce empty makefile if lib/cucumber/messages_pb.rb exists

This should avoid trouble when installing, for example if bash is not available.
The makefile should only be used during development, for released version the file
is there and nothing should be done.

Author: vincent-psarga

cucumber-messages/ruby/ext/cucumber-messages/extconf.rb

@@ -1,9 +1,24 @@
 require "mkmf"
 
 find_executable('protoc')
+makefile_content = ''
 
-File.open(File.join(Dir.pwd, 'Makefile'), 'w') do |file|
-  file.write %Q(
+if File.exist?('../../lib/cucumber/messages_pb.rb')
+  makefile_content = %Q(
+default:
+\t@echo "Nothing to do"
+.PHONY: clean
+
+install:
+\t@echo "Nothing to do"
+.PHONY: clean
+
+clean:
+\t@echo "Nothing to do"
+.PHONY: clean
+)
+else
+  makefile_content = %Q(
 SHELL := /usr/bin/env bash
 
 default: ../../lib/cucumber/messages_pb.rb
@@ -21,4 +36,8 @@
 )
 end
 
+File.open(File.join(Dir.pwd, 'Makefile'), 'w') do |file|
+  file.write makefile_content
+end
+
 $makefile_created = true