GTMSessionFetcher cannot be integrated as static libraries with CocoaPods because it does not define a module (`DEFINES_MODULE`)

[jmagman]

Steps to reproduce

1. Install CocoaPods.
2. In Xcode, make new iOS app project.
3. cd into the directory and pod init to initialize CocoaPods.
4. In the generated Podfile comment out use_frameworks! to embed as static libs. Add a pod that depends on GTMSessionFetcher, for example, GTMAppAuth:

  # Comment the next line if you don't want to use dynamic frameworks
  # use_frameworks!
  pod 'GTMAppAuth', '4.1.1'

5. Run pod install

Result
pod install fails:

$ pod install
Analyzing dependencies
Downloading dependencies
Installing AppAuth (1.7.3)
Installing GTMAppAuth (4.1.1)
Installing GTMSessionFetcher (3.3.2)
[!] The following Swift pods cannot yet be integrated as static libraries:

The Swift pod `GTMAppAuth` depends upon `GTMSessionFetcher` and `AppAuth`, which do not define modules. To opt into those targets generating module maps (which is necessary to import them from Swift when building as static libraries), you may set `use_modular_headers!` globally in your Podfile, or specify `:modular_headers => true` for particular dependencies.

GTMSessionFetcher is not setting up a module, and doesn't have DEFINES_MODULE for that to be done automatically.

This can be fixed by adding DEFINES_MODULE to GTMSessionFetcher.podspec. See CocoaPods documentation about modular headers.

  s.xcconfig = {
     'DEFINES_MODULE' => 'YES',
  }

[thomasvl]

@paulb777 - doesn't seem like you all have to do this for Firebase? This sorta feels like a "down stream" need that shouldn't be pushed into the .podspec here. Anyway, thoughts?

[paulb777]

Because of the transition to Swift implementations beginning in Firebase 9.0, Firebase now requires use_frameworks! or use_frameworks! :linkage => :static in Podfiles.

[thomasvl]

So safe to add here without impacting folks now using Swift and/or modules?

[paulb777]

Without testing, I would guess so, since modules should always be built for frameworks. cc: @mdmathias