Simplify/fix libffi_feature_test As detailed in <http://news.gmane.org/find-root.php?message_id=%3C87wppswqqt.fsf%40kepler.schwinge.homeip.net%3E>, the original code (originally added in commit f1301a54bb80e6ae23f7687c68f36875dae69134 as part of pull request #145) does not work (at least not for me, in the GCC environment).