I fixed the problem and it has to do with some updates in the Android L platform that is only available in the 64-bit Android NDK r-10.
- The NDK has dropped atomics.h for the Android L platform in favor of <stdatomic.h> (source: https://code.google.com/p/android/issues/detail?id=73744). So you can copy atomics.h from the 32-bit NDK into your ndk-r10\platforms\Android-L\<every arch you want to support>\usr\include\sys\
- Also for NDK-r10, "the sys/timeb.h header has been removed in the next platform version (currently available in NDK r10 in the "L preview" release), since this header and the ftime function have been removed from the POSIX 2008 specification." (source: https://rbcommons.com/s/OpenH264/r/683/). So you can copy timeb.h from the 32-bit NDK into the same place as atomics.h, mentioned above.
That worked for me! It's a bit combersome but I'm guessing it will be fixed eventually on the JUCE side. It must suck constantly having to update the code base to work on all the changing SDKs, platforms, etc. God speed man.
Now I'm going to try it for 64-bit iOS 7.1. Fingers crossed.