Use 'mkdir -p' to create parent directories as needed. Prevents this error:

  $ make multiple-html
  mkdir result/en/out-htmls
  mkdir: cannot create directory `result/en/out-htmls': No such file or directory

git-svn-id: http://svn.voria.com/code@282 1f10aa63-cdf2-0310-b900-c93c546f37ac