Fix building of JSC for earlier versions of gcc on Mac

Since building for Carbon typically uses older versions of gcc on older
versions of Mac, then we need to revert
0de22e80ac645afc3793419300d6271d95809196 partially in order to fix this.
Since the original code was just a cleanup then this is safe to do so.

Change-Id: I5dd8fe99704ffac66ac3384b62849fa8162ca82d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Qt 3 Support: Prevent double deletion of QObjectPrivate::threadData in QSettings.

Task-number: QTBUG-36908

Change-Id: I8f9f86f20bd8bb479c2637f6c276e4e2593ecef9
Reviewed-by: Andy Shaw <andy.shaw@digia.com>