gc.c: refine macros
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41445 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
gc.c: refine macros
used as boolean value.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41445 b2dd03c8-39d4-4d8f-98ff-823fe69b080e