qmake: Load extra variables only if also loading pre-files

The extra variables only need to be applied once, when we
are loading the pro file (and hence are loding pre files),
not for every single pri/prf that's loaded as a result of that
(which do not load pre files themselves).

Change-Id: I3118694a8eeccf2dc32c4f62df754033fad13528
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/8e7dc25380dceebca094e092d9feb21ad167ba91)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>