Error: "File was built for archive which is not the architecture being linked (armv7s)"
I ran into a similar problem myself. @graver's solution is definitely valid.
The issue was that the library was being built for armv7 instead of armv7s. You can verify this yourself by using lipo <path/to/lib.a> -info.
Setting the Build Active Architectures Only option to No fixes the issue.
Hope this helps.
When you're building a library you must compile it both for the simulator and the device and then merge the two outputs (.a files) into 1 library and then link it to your iOS project.
one additional notes is:
Build Active Architectures Only set to nois for the lib project.