SDL_mixer : Specify branch SDL-1.2 for download 
SDL_mixer : Specify branch SDL-1.2 for download 

https://github.com/kivy/kivy-ios/issues/19

Default SDL_mixer branch now wants to build version 2, which causes both the SDL_mixer build to fail, and consequently, the kivy build as well.

Xcode Frameworks will show libSDL_mixer.a and libkivy.a are missing.

Specifying the SDL-1.2 branch results in the build working, fixing both problems.

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