Add DEFINES_MODULE to support downstream usage of static libs

[thomasvl]

Fixes #384

[thomasvl]

@paulb777 @mdmathias - this seem good?

[paulb777]

I can stage the modified podspec to https://github.com/firebase/SpecsStaging and make sure it doesn't impact Firebase tests - and let you know tomorrow.

[paulb777]

I staged a version (without visionOS) at firebase/SpecsStaging@948fd05 cc:@ncooke3

[mdmathias]

I'm going to be making a similar change for AppAuth-iOS.

[paulb777]

Firebase nightly tests look good with the gtm-session-fetcher podspec update - firebase/firebase-ios-sdk#12669

[thomasvl]

Thanks, merging.

[vonovak]

Hello and thanks for this fix 🙂

@thomasvl would it be possible to have this released soon?

There are downstream packages building on top of this, such as wrappers for Google Sign in SDK for React Native and maybe Flutter and others that are hitting #384 and having this published would make it easy to use the latest versions.

I'm aware that there are workaround that are not too complicated even, but requiring people to take extra steps, however easy and nicely documented means friction and questions in issue tracker 🙈 .

Thank you 🙂