[ruby/cgi] Set extconf.rb to extensions
Fix https://github.com/ruby/cgi/pull/11
https://github.com/ruby/cgi/commit/60d8f5e7d9
[ruby/cgi] Set extconf.rb to extensions
Fix https://github.com/ruby/cgi/pull/11
https://github.com/ruby/cgi/commit/60d8f5e7d9