解决OBJC_CLASS_$_MBProgressHUD无法引用的问题（转载）

Undefined symbols for architecture i386:

  "_OBJC_CLASS_$_MBPRogressHUD", referenced from:

      objc-class-ref in ViewController.o

ld: symbol(s) not found for architecture i386

clang: error: linker command failed with exit code 1 (use -v to see invocation)