Added s flag to ar command

src/mruby_core.rake

@@ -6,7 +6,7 @@ MRuby.each_target do
   self.libmruby << objs
 
   file "#{build_dir}/lib/libmruby_core.a" => objs do |t|
-    archive t.name, 'r', t.prerequisites
+    archive t.name, 'rs', t.prerequisites
   end
 
   # Parser

tasks/libmruby.rake

 MRuby.each_target do
   file "#{build_dir}/lib/libmruby.a" => libmruby.flatten do |t|
-    archive t.name, 'r', t.prerequisites
+    archive t.name, 'rs', t.prerequisites
   end
 end