configure.in: off_t for struct stat.st_size

* configure.in (struct stat.st_size): prefer off_t over int, long,
  and so on.  inspired by
  http://www.opensource.apple.com/source/ruby/ruby-104/patches/config.h.ed

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47825 b2dd03c8-39d4-4d8f-98ff-823fe69b080e