I found a few answers for doing it in code files, but my warning is coming from interface builder, and it is silly.
Attribute unavailable: Defines Presentation Context is not available prior to Xcode 4.2.
I am using Xcode 4.2. Why would I possibly care about this warning? Lots of things don’t work in Xcode 0.01. This is day one of a new project and I don’t want to spend the whole time violating the no-checking-in-with-warnings rule.
Select xib file; in the Utilities pane (on the right), under File Inspector tab, find “Interface Builder Document” section; change the Development property to “Default Version (Interface Builder 4.2)”.