Introduce Qt.core.config property for user-config items.

Change-Id: I051daa84a6a8e569a13e8ff289563dfc7df77853
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Set compiler flags for using a libc++ build of Qt.

Task-number: QBS-194

Change-Id: Ic74aa7d53df0ff25bbaf8ce4d599adbe00dbbba4
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>