LDP: Update Makefile

LDP: Update Makefile

* When a new category is added in perkamon,
  po4a/<category>/po/ja.po also needs to be created
  in addition to po4a/<category>/po directory.
  Otherwise, "make translate-pages" (and "make stamp/<category>")
  do not work.
* Call "make clean" in perkamon rather than deleting files in
  perkamon manually. "perkamon/po4a/*/po" need to be deleted,
  but "make clean-setup" did not handle it previously.
* Sort po4a modules alphabetically to ensure po4a modules are
  processed in the expected order.