update to iOS 9.2 SDK and define sysroot for iOS
update to iOS 9.2 SDK and define sysroot for iOS

Previously, we only seemed to define sysroot for MacOS, not iOS, yet
we reference $(sysroot) in both cases.  This ensures it is defined in
both cases as well.

lwahlmeier/avian,dicej/avian,lwahlmeier/avian,MaartenR/avian,joshuawarner32/avian,lwahlmeier/avian,MaartenR/avian,joshuawarner32/avian,joshuawarner32/avian,dicej/avian,MaartenR/avian,MaartenR/avian,joshuawarner32/avian,dicej/avian,dicej/avian,lwahlmeier/avian