fix symlink installation

Before overwriting an existing symlink we must remove it.
Otherwise the symlink() call fails.

Task-number: QBS-391
Change-Id: I9cd0cdc5c71eea9e4680c8b53a780e1c5c2ab2c2
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
setup Qt's run environment on Windows only

Change-Id: I2e90d380615b825210f7830bfc37c16c013b469c
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>