Setting QT_SELECT globally via debian-qt-kde.mk v3

[Rohan Garg]

Hi
We're currently facing a general problem that a few frameworks
packages explicitly export QT_SELECT in their debian/rules (
kdesignerplugin, kjobwidgets, kauth to name a few ).

Since v3 of debian-qt-kde.mk only supports Qt5 and Frameworks 5
anyway, I was wondering if I could just go ahead and export
QT_SELECT=5 directly in debian-qt-kde.mk instead of each packaging
doing it for themselves.

Since this only affects v3 of the script, KDE4 packages using v2
should be un affected by this change.

Cheers
Rohan Garg