qt: ensure cross pkg-config is used

Closes #4297

Tweak qmake.conf to ensure our cross pkg-config is used, instead of
whatever is first in the path. In order to do this, tweak the
QT_QMAKE_SET to be able to handle variables which aren't prefixed with
QMAKE_.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>