Replace Q_WS_MAC with Q_OS_MAC.

We want to carry over all mac-specific code paths
to Qt 5. WS_MAC is no longer set; OS_MAC is.

This change is compatible with Qt 4 builds since
OS_MAC is set whenever WS_MAC is. (OS_MAC would
also be set for the theoretical X11/Mac platform,
but that's anyway not a supported configuration
for Creator)

Change-Id: Ib25d8c4e83e7e27d911124cd48aa17fdb875923f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>