I am trying to integrate unity to iOS application. I have followed this tutorial http://www.agnosticdev.com/blog-entry/swift/integrating-unity-and-vuforia-ios-swift-project
I had the same issue and I solve it adding to Apple LLVM - Custom Compiler FLags:
Apple LLVM - Custom Compiler FLags
Other C Flags: -DRUNTIME_IL2CPP=1