* variable.c (rb_const_get_0): warn deprecated constant reference.
* variable.c (rb_mod_deprecate_constant): mark constants to be
  warned as deprecated.  [Feature #11398]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51444 b2dd03c8-39d4-4d8f-98ff-823fe69b080e