在arc模式下，使用weak修饰符时clang -rewrite-objc xxx.m时会提示编译错误：

cannot create __weak reference because the current deployment target does
      not support weak references
    __attribute__((objc_ownership(weak))) NSObject *obj2 = obj;

解决方法：

clang -rewrite-objc -fobjc-arc -stdlib=libc++ -mmacosx-version-min=10.7 -fobjc-runtime=macosx-10.7 -Wno-deprecated-declarations main.m