Eu adicionei ao meu arquivo pod

pod 'google-plus-ios-sdk', '1.4.1'

Eu fiz um

pod update

Eu importei

#import <GooglePlus/GooglePlus.h>

Mas se eu tentar

[GPPSignIn sharedInstance].clientID = kClientID; 

Eu recebo este erro do linker:

Undefined symbols for architecture i386:
"_OBJC_CLASS_$_GPPSignIn", referenced from:
      objc-class-ref in SDSocialManager.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)