Merge pull request #4779 from take-cheeze/print_mrbc_cmd

Print mrbc command on verbose mode

lib/mruby/build/command.rb

@@ -327,7 +327,9 @@ module MRuby
       infiles.each do |f|
         _pp "MRBC", f.relative_path, nil, :indent => 2
       end
-      IO.popen("#{filename @command} #{@compile_options % {:funcname => funcname}} #{filename(infiles).join(' ')}", 'r+') do |io|
+      cmd = "#{filename @command} #{@compile_options % {:funcname => funcname}} #{filename(infiles).join(' ')}"
+      print("#{cmd}\n") if $verbose
+      IO.popen(cmd, 'r+') do |io|
         out.puts io.read
       end
       # if mrbc execution fail, drop the file