Use the qmake variable for -ldl on Unix systems

There are two important benefits in using this variable:
 * on systems where the library isn't libdl, we now get it right
   (HP-UX, for example, has libdld).
 * on systems where the library isn't necessary, we won't add anything
   (the BSDs, including Darwin)

Change-Id: Ibf8155fd664b426165e55a0b95bf6a2c33dbf1d0
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>