ifchange: no overwrite with an empty file

* tool/ifchange, win32/ifchange.bat: do not overwrite with an empty
  file by default, and add --empty option to force it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43519 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
gc.c: use boolean

* gc.c (obj_free): use free_immediately as boolean.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43520 b2dd03c8-39d4-4d8f-98ff-823fe69b080e