Call default_pre before using its results

To check the XCode version, default_pre has to be run first.
Under most circumstances the order is not important since the value
gets cached, that's why current builds usually work.

Change-Id: I2613e92189592d7b25200e22a378f9f593b5057e
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>