Cocoapods: unable to set pod build setting - xcode

Question

In my current project, I'm trying to include an in-house pod with two subspecs - one for the regular app, one for the today widget extension. Unfortunately, Cocoapods (1.2.1) is wrongly inferring the build flags for the pod target. Both targets - the extension pod target and the normal app pod target - have the "Require only App-Extension-Safe API" set to YES, resulting in the -fapplication-extension CLANG compiler flag. Sadly, one target doesn't build in this configuration.
Manually setting the flag to NO resolves the issue, but only until the next pod update. To be specific, this is the setting I'm trying to address:
I'm not even sure what heuristic Cocoapods is using to infer wether this needs to be turned on or off, but is there a way to explicitly set the target build setting? I tried
s.subspec 'Core' do |core|
core.dependency 'xxxxxxx','~> 1.0.1'
core.source_files = 'Pod/Sources/**/*.{h,m,mm}'
core.ios.xcconfig = {
'CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES' => 'YES',
'APPLICATION_EXTENSION_API_ONLY' => 'NO'
}
core.tvos.xcconfig = {
'GCC_PREPROCESSOR_DEFINITIONS' => 'TVOS=1',
'CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES' => 'YES'
}
end
and
s.subspec 'Core' do |core|
core.dependency 'xxxxxxx','~> 1.0.1'
core.source_files = 'Pod/Sources/**/*.{h,m,mm}'
core.pod_target_xcconfig = { 'APPLICATION_EXTENSION_API_ONLY' => 'NO' }
core.ios.xcconfig = {
'CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES' => 'YES',
}
core.tvos.xcconfig = {
'GCC_PREPROCESSOR_DEFINITIONS' => 'TVOS=1',
'CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES' => 'YES'
}
end
Unfortunately, both to no avail. The setting remains stubbornly at YES. I'm at my wit's end here. Anything I can try?

Answer 1

Self-answering: I still don't know how to force the setting, but the problem was actually in our (multi-tiered) dependency graph. A depended-upon subspec was used by both the app and the app extension target, hence forcing the compiler flag for that subspec target.
I fixed it by adding yet another subspec so that the pod target isn't shared by the app and the extension.