Fix check for qbs library type.

Qt 5 no longer guarantees that "shared" or "static" appear in the CONFIG
variable. So assume "shared" whenever "static" is not set.

Change-Id: I9f9f71d4aeacd4c083842a01458fdd25a9980523
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>