If you build a build error in Xcode
You can view these logs via a tool built into Apple OS.
Cd to the above directories and open the log files
> cd ~/Library/Logs/DiagnosticReports
> open ruby_xxx.crash
You can then see from above code that you fail in ruby string.c land. Ruby version error.
This program is called the Console application and you can find it in your applications folder.