App Extension compatibility in Podspec

[tobihagemann]

Fixes #432

It's really an old issue but I think everyone can benefit from this small change when dealing with CocoaPods and App Extensions. Using subspecs is basically "how it should be done", see: CocoaPods/CocoaPods#5373 (comment)

[nab0y4enko]

@tobihagemann @juliensaad Can you check this PR?

[tobihagemann]

For anyone still wondering what this change is about: It's great to have the SV_APP_EXTENSIONS preprocessor macro so that this library can be used with app extensions but when using CocoaPods it's not that trivial to set that macro (at least to my knowledge and as discussed in the linked issues).

With the change in this PR, you'd be able to define the following in your Podfile:

target 'MainApp' do
  pod 'SVProgressHUD'
end

target 'AppExtension' do
  pod 'SVProgressHUD/AppExtension'
end

That's it. You do not have to worry about preprocessor macros at all. And as you can see, it doesn't change the current behavior because the newly defined Core subspec is the default subspec.