编写category时的便利宏（用于解决category方法从静态库中加载需要特别设置的问题）

代码摘录自YYKit:https://github.com/ibireme/YYKit

/**
 Add this macro before each category implementation, so we don't have to use
 -all_load or -force_load to load object files from static libraries that only
 contain categories and no classes.
 More info: http://developer.apple.com/library/mac/#qa/qa2006/qa1490.html .
 *******************************************************************************
 Example:
     YYSYNTH_DUMMY_CLASS(NSString_YYAdd)
 */
#ifndef YYSYNTH_DUMMY_CLASS
#define YYSYNTH_DUMMY_CLASS(_name_) \
@interface YYSYNTH_DUMMY_CLASS_ ## _name_ : NSObject @end \
@implementation YYSYNTH_DUMMY_CLASS_ ## _name_ @end
#endif