To find out what's wrong here, the best bet is to run the Ruby installation yourself to see exactly what's amiss. ruby-build typically downloads and extracts Ruby in /tmp (it should be the same path where it also saves the log file of the failed install).
Go to that directory and run:
$ ./configure
$ make
To see if you get the same error. If you do, then try to re-run the command that failed again. In your case this is probably:
$ ranlib libruby-static.a
To see why it fails.