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.

git-svn-id: https://svn.apache.org/repos/asf/perl/modperl/trunk@1457512 13f79535-47bb-0310-9956-ffa450edef68