Since we're only including the Extras file one time, invoking set() for
the include dirs again will overwrite the addition of include dirs in
the extras file.
We only need to populate these variables if not set anyway, so do that.
Change-Id: I04dad0674778e79c8c12c18231b8ce6c92edf881
Reviewed-by: Alexander Neundorf <neundorf@kde.org>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>