fix bugs on building extensions

Rakefile

@@ -9,10 +9,14 @@ end
 
 task build: :compile
 
-%w[decoders/native decoders/crunch].each do |dir|
+ext_dirs = %w[decoders/native decoders/crunch]
+
+ext_dirs.each do |dir|
   Rake::ExtensionTask.new(dir) do |ext|
     ext.lib_dir = 'lib/mikunyan'
   end
 end
 
+task compile: ext_dirs.map{|e| "compile:#{e}".to_sym}
+
 task default: %i[clobber compile spec]