Merge pull request #5155 from shuujii/refactor-mrblib-mrblib.rake

Refactor `mrblib/mrblib.rake`

mrblib/mrblib.rake

 MRuby.each_target do
-  current_dir = File.dirname(__FILE__)
-  relative_from_root = File.dirname(__FILE__).relative_path_from(MRUBY_ROOT)
-  current_build_dir = "#{build_dir}/#{relative_from_root}"
+  src = "#{build_dir}/mrblib/mrblib.c"
+  obj = objfile(src.ext)
+  rbfiles = Dir["#{MRUBY_ROOT}/mrblib/*.rb"].sort!
 
-  self.libmruby_objs << objfile("#{current_build_dir}/mrblib")
+  self.libmruby_objs << obj
 
-  file objfile("#{current_build_dir}/mrblib") => "#{current_build_dir}/mrblib.c"
-  file "#{current_build_dir}/mrblib.c" => [mrbcfile, __FILE__] + Dir.glob("#{current_dir}/*.rb").sort do |t|
-    _, _, *rbfiles = t.prerequisites
+  file obj => src
+  file src => [mrbcfile, __FILE__, *rbfiles] do |t|
     mkdir_p File.dirname(t.name)
-    open(t.name, 'w') do |f|
-      _pp "GEN", "*.rb", "#{t.name.relative_path}"
+    File.open(t.name, 'w') do |f|
+      _pp "GEN", "mrblib/*.rb", "#{t.name.relative_path}"
       f.puts %Q[/*]
       f.puts %Q[ * This file is loading the mrblib]
       f.puts %Q[ *]