- The subdir argument was not really used.
  - Previously get_lib_directory was called with either "dia"
    or "locale" as subdir:
      - the subdir=="locale" call was removed by rewriting
        dia_get_locale_directory.
      - next, the subdir argument was removed and instead of
        appending "dia" to LIBDIR, we just use DIALIBDIR.
  - Define DIALIBDIR as simply $(libdir)/dia,
    without having to define LIBDIR as well.