The extconf.rb file in scripts/ruby adds an LDFLAG (--flat-namespace) which is specific to Darwin (man gcc). Subsequent compilation therefore fails on non-Darwin machines when ruby bindings are enabled. This patch isolates the offending flag to Darwin machines and addresses previously reported bug ID 3495959
Thanks! Sorry I didn't see the new patch tracker. It's now in the Git repo (http://github.com/openbabel/openbabel).