[osx/ios/atv2] - fix missing _DEBUG and NDEBUG defines due to missing in...

...heritance in the target level xcconfig files.

We need to inherite the GCC_PREPROCESSOR_DEFINITIONS from Debug.xcconfig/Release.xcconfig (project level) in the App-*.xcconfig files (target level). Else we are not getting the _DEBUG and NDEBUG flags set on the target (basically we had them overwritten in the target).

thx to @karlson2k for pointing me to the flaw

Gotham material ...

@diederich for review