The modern approach to using SDKs in Xcode is to build against the latest SDK and set the deployment target to the oldest version on which you want to deploy. We're currently always trying to build against the SDK corresponding to the oldest version on which we want to deploy. This is problematic with newer versions of Xcode that don't include older SDKs.
Created attachment 151796 [details] Patch v1
Landed in r122404.