[Qt] Enable thin archives before parsing the project files

Rubber-stamped by Simon Hausmann.

The targetSubDir() function can be called before default_post.prf has been
parsed therefore the gnu_thin_archives option has to be set in default_pre.prf
which is parsed before the main project file.
This issue was revealed by r144299 and fixes the clean build.

* qmake/mkspecs/features/default_post.prf:
* qmake/mkspecs/features/default_pre.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144306 268f45cc-cd09-0410-ab3c-d52691b4dbfc