Para el proyecto en el que estoy trabajando, necesitaba descomprimir ciertos archivos. Para esto, encontré la biblioteca SSZipArchive. Incluí esto en Xcode 4.2 (haga clic con el botón derecho en la carpeta Clases-> Agregar archivos al proyecto, con la casilla de verificación "Copiar elementos en la carpeta del grupo de destino" en el diálogo). Incluyo la biblioteca libz (he probado tanto libz como zlib1.2.5). Intento compilar y de repente tengo 20 errores:

/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:311:1: error: expected identifier or '(' [1]
 @class NSString, Protocol;
 ^    
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:313:19: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT NSString *NSStringFromSelector(SEL aSelector);
                   ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:314:44: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT SEL NSSelectorFromString(NSString *aSelectorName);
                                            ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:316:19: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT NSString *NSStringFromClass(Class aClass);
                   ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:317:43: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT Class NSClassFromString(NSString *aClassName);
                                           ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:319:19: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT NSString *NSStringFromProtocol(Protocol *proto) NS_AVAILABLE(10_5, 2_0);
                   ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:319:50: error: unknown type name 'Protocol' [1]
 FOUNDATION_EXPORT NSString *NSStringFromProtocol(Protocol *proto) NS_AVAILABLE(10_5, 2_0);
                                                  ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:320:19: error: unknown type name 'Protocol' [1]
 FOUNDATION_EXPORT Protocol *NSProtocolFromString(NSString *namestr) NS_AVAILABLE(10_5, 2_0);
                   ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:320:50: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT Protocol *NSProtocolFromString(NSString *namestr) NS_AVAILABLE(10_5, 2_0);
                                                  ^
 /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:324:30: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT void NSLog(NSString *format, ...) NS_FORMAT_FUNCTION(1,2);
                              ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:324:53:{324:53-324:76}: error: format argument not an NSString [3]
 FOUNDATION_EXPORT void NSLog(NSString *format, ...) NS_FORMAT_FUNCTION(1,2);
                                                     ^~~~~~~~~~~~~~~~~~~~~~~
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:85:49: note: instantiated from:
         #define NS_FORMAT_FUNCTION(F,A) __attribute__((format(__NSString__, F, A)))
                                                        ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:325:31: error: unknown type name 'NSString' [1]
 FOUNDATION_EXPORT void NSLogv(NSString *format, va_list args) NS_FORMAT_FUNCTION(1,0);
                               ^
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:325:63:{325:63-325:86}: error: format argument not an NSString [3]
 FOUNDATION_EXPORT void NSLogv(NSString *format, va_list args) NS_FORMAT_FUNCTION(1,0);
                                                               ^~~~~~~~~~~~~~~~~~~~~~~
/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:85:49: note: instantiated from:
         #define NS_FORMAT_FUNCTION(F,A) __attribute__((format(__NSString__, F, A)))

Estos son todos los errores en la biblioteca principal, por lo que probablemente algo salió mal al incluir la biblioteca en Xcode. Logré reducir al culpable a la biblioteca de minizip que utiliza SSZipArchive (si elimino esa biblioteca, los errores desaparecen y el compilador se ejecuta como debería), pero estoy bastante perplejo sobre por qué esto hace que el compilador tenga tantos problemas .