bigdecimal.c: backword compatibility as gem

* ext/bigdecimal/bigdecimal.c (CLASS_NAME): macro to wrap
  depending on PRIsVALUE for 1.9.  [Backport #9406]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e