[Qt] Only use thin archives on Linux

Rubber-stamped by Jocelyn Turcotte.

Since "thin archive" is a feature of GNU's ar
we limit the usage to Linux to avoid problems
with other configurations (eg. macx-g++).

* qmake/mkspecs/features/default_pre.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148744 268f45cc-cd09-0410-ab3c-d52691b4dbfc