I downloaded the QT4 package for you ditribution OpenSuse 12.1 and
read the changelog and it shows that this is a new feature in QT4.8
and I am still on 4.7.
I checked this point using Qt 4.7.2 in another machine of mine. Indeed,

qmake -unset VARIABLE

was not known then. However, this statement works for me in Qt 4.7.2:

qmake   -set PREFIX ""