this is an exclusively user-controllable variable. the mkspec may provide
a default via QT_QPA_DEFAULT_PLATFORM when it deviates from the platform
default known to configure, and these specs actually already make use of
that.
Change-Id: I7531063d4c404ebed7b2cc1647c61626db30daef
Reviewed-by: Kyle Edwards <kyle.edwards@kitware.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>