support response files in GCC toolchain

GCC supports response files with the prefix @ on all platforms.

Task-number: QBS-276
Change-Id: Ib4b310bce95e51298f1eb81c2cdac96791589f9d
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
fix overrideProjectProperties test for OS X

Change-Id: I6ed287f4e696d48d704f19b49ff9f9283f5e7084
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>