swift-nio: CNIOZlib Xcode build error

Expected behavior

The ability to build the latest SwiftNIO release from within Xcode.

Actual behavior

I’m getting a build error Include of non-modular header inside framework module 'CNIOZlib': '/opt/local/include/zlib.h' inside the CNIOZlib.h file at #include <zlib.h>.

Steps to reproduce

1. Download the latest release
2. Run swift package update; swift package generate-xcodeproj
3. Open the generated Xcode project, and try building the swift-nio-Package schema.

SwiftNIO version/commit hash

Release 1.10.0

Notes

It seems that swift build --target NIO builds successfully; this problem exists solely when building from Xcode.

I think https://github.com/apple/swift-nio/commit/9bae007a15b49d7fe685b6b33308822c6d0536c0 is the culprit, as I can build successfully from the previous commit, https://github.com/apple/swift-nio/commit/ed28803a78b187e202c27a62c7a497fbe9cfbbd7.

I am running MacOS 10.14.1 Beta (18B73a).