twmn: Fix running of {pre,post}Configure

Commit 0055c6aa

 introduced a new preConfigure hook that sets the right
qmake path. Unfortunately the mkDerivation attributes of twmn override
the whole configurePhase, so this hook isn't run at all.

This fixes the build of twmn and it now successfully compiles on my
machine.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>