By default, React Native projects target a minimum of iOS 7, but our projects were set to iOS 8 because previously we used a dynamic framework.
When building for an iOS 7 minimum target version, the linker would complain about thread local variables not working on iOS 7. So it was necessary to add even more checks (iOS >= 8 or OS X >= 10.7) before using native thread local variables rather than our shim.
This also changes our projects to target iOS 7 so we can catch these issues quicker in the future.
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