I have built my own Static C++ Library, which is built with the settings:
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.