Don't use QT_BUILD_TREE anymore

Don't use QT_BUILD_TREE anymore

Modules have their own variables to use for such things
If there's no way of getting to what you need outside of those,
you probably shouldn't be doing it in the first place.

Change-Id: I3e8d5f632ea377ec5dcb3eb0fe7dd27d2f4deb90
Reviewed-on: http://codereview.qt.nokia.com/1850
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>