Support regeneration of ja-dic.el under '--with-small-ja-dic'

Support regeneration of ja-dic.el under '--with-small-ja-dic'

* lisp/international/ja-dic-cnv.el (skkdic-convert): Record the
value of SMALL_JA_DIC option used to produce ja-dic.el.
* leim/Makefile.in (small-ja-dic-option): New target, triggers
regeneration of ja-dic.el when the value of SMALL_JA_DIC option
changes by the configure script.  Suggested by Ulrich Mueller
<ulm@gentoo.org>.
(${leimdir}/ja-dic/ja-dic.el): Depend on 'small-ja-dic-option'.
(Bug#66125)

* make-dist (possibly_non_vc_files):
* .gitignore: Add 'leim/small-ja-dic-option'.