Caution ! There is a bug at latest XCode 4.3.1 !
Don’t use the dragging files to add your source files.
Sometimes the XCode can’t recognize it as a source file, and you will get an error when you link your .o files.
You need to use the add file features instead.
This post is also on http://blog.molioapp.com/2012/04/note-about-latest-xcode-431.html