Para o projeto em que estou trabalhando, precisava descompactar determinados arquivos. Para isso, encontrei a biblioteca SSZipArchive. Incluí isso no Xcode 4.2 (clique com o botão direito do mouse na pasta Classes-> Adicionar arquivos ao projeto, com no diálogo a caixa de seleção "Copiar itens na pasta do grupo de destino"). Eu incluo a biblioteca libz (eu tentei a libz e o zlib1.2.5). Tento compilar e de repente tenho 20 erros:

/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)))

Estes são todos os erros na biblioteca principal, então provavelmente algo deu errado ao incluir a biblioteca no Xcode. Eu consegui restringir o culpado à biblioteca minizip que o SSZipArchive usa (se eu remover essa biblioteca, os erros desaparecem e o compilador é executado como deveria), mas estou perplexo em saber por que isso faz com que o compilador tenha tantos problemas .