Set APPLICATION_EXTENSION_API_ONLY to Pods when set in the project's .xcconfig

[juanjonol]

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Since pull request #4321, the APPLICATION_EXTENSION_API_ONLY build setting is automatically applied to Pods if it's set in the user target. But this doesn't work if APPLICATION_EXTENSION_API_ONLY is set in an Config file (.xcconfig).

This also applies to the BUILD_LIBRARY_FOR_DISTRIBUTION Build Setting if #9232 is fixed.

What did you do?

1. Create a new project.
2. Create an .xcconfig file and make the project use it.
3. Add APPLICATION_EXTENSION_API_ONLY = YES to the .xcconfig.
4. Use pod install to install any pod.

What did you expect to happen?

CocoaPods should set APPLICATION_EXTENSION_API_ONLY to YES in all Pods, like it does if the setting is applied directly to the project.

What happened instead?

Pods don't have the APPLICATION_EXTENSION_API_ONLY set to YES, so there's a Linking against a dylib which is not safe for use in application extensions warning for each Pod.

Workaround

Use a post_install hook to set APPLICATION_EXTENSION_API_ONLY to YES in all pods.

[dnkoutso]

This PR will allow for this to be fixed.