json_index.rb: check zlib
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50059 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
json_index.rb: check zlib
unless zlib is available.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50059 b2dd03c8-39d4-4d8f-98ff-823fe69b080e