RUBY-530 sasl gssapi support for MRI

We currently support kerberos authentication for JRuby.
This pull request adds a c extension that allows you to use Kerberos when on MRI.
Is this normal in ruby to do these "if not ok - otherwise"?

My brain strongly prefers if (true) {} else {/* failure branch*/ } over if (not true) { /* failure branch */ } else { }

So I personally would have done:
if (sasl_client..() == SASL_OK) {
   rb_iv_set(..... true);
   /* and all the other variables */
} else {
/* set it to false */
}