mkconfig.rb: reconstruct comma separated list values

* tool/mkconfig.rb: reconstruct comma separated list values.  a
  command line to Windows batch file is splitted not only by spaces
  and equals sign but also by commas and semicolons.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39507 b2dd03c8-39d4-4d8f-98ff-823fe69b080e