Fix for iOS7 deployment when using iOS 8 SDK

The following check only ensures that it is being built with iOS8 SDK.
__IPHONE_OS_VERSION_MAX_ALLOWED >= 80000
But if we build with iOS 8 SDK and try to deploy it on iOS 7 or 7.1 we get a linker error. 
This can be avoided by adding the deployment target check as done in this commit.