Restore build with Perl 5.8.x (although 5.8.0 still has other problems for me). - Recent perls set $Config{usethreads} to 'define' when built with ithreads, but older perls did not (-Dusethreads was different to -Duseithreads), causing a perl version check failure when wrongly testing $Config{usethreads} instead of the intended $Config{useithreads}. - Newx and Newxz were added in 5.9.3.