title: Undefined symbols for architecture x86_64 on Xcode 6.1
body: <p>All of a sudden Xcode threw me this error at compilation time: </p>
  
  <pre><code>Undefined symbols for architecture x86_64:
  "_OBJC_CLASS_$_Format", referenced from:
   objc-class-ref in WOExerciseListViewController.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>After doing some research, I may understand that a library I'm using is not compatible with 64 bit version. But this is very strange since I've been working with the same libraries for at least a week without having a single compilation problem during that time. The two libraries are just composed of a bunch of classes, and when I removed them from my project I got the same issue. Since I have never created libraries myself, I have no idea how I can find wether the ones I'm using are compatible with 64 bit (?)
  I also tried the following changes for architectures under the target panel : </p>
  
  <ul>
  <li>added $(ARCHS_STANDARD_INCLUDING_64_BIT) to the Architectures</li>
  <li>build active architecture only -> set to 'NO' </li>
  <li>for 'valid architectures' -> set to arm64, armv7 and armv7s</li>
  <li>deleted the DerivedData folder and its contents, cleaned and built again </li>
  </ul>
  
  <p>But none of these changes work. Please, does someone have a clue on this? 
  Thanks </p>