Support immediate commit on postfix kanji2seq conversion.

* scm/tutcode.scm
  - (tutcode-begin-postfix-kanji2seq-conversion):
    If yomi-len argument is 0, treat as #f and commit immediately.
configure.ac: fix a check of debug flag for #qt3

$enable_debug is either "yes" or "no", so "test -z" is always false