dyld: lazy symbol binding failed: Symbol not found: _objc_setProperty_nonatomic_copy

工程跑SDK6.1的时候很正常，但今天跑了一下SDK5.1就出现问题了：

dyld: lazy symbol binding failed: Symbol not found: _objc_setProperty_nonatomic_copy

  Referenced from: /Users/xxx/Library/Application Support/iPhone Simulator/5.1/Applications/8BB5CBD7-DAE9-4189-B76F-E682BC95746F/xxx.app/xxx

  Expected in: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/

  iPhoneSimulator5.1.sdk/System/Library/Frameworks/Foundation.framework/Foundation

一开始不知道问题出在了哪里，找了半天才发现是静态库的target不对，只要将静态库的target改成5.1，再跑5.1的时候就好了。