ios-build: use the macros in TargetConditionals.h

- Replaces the TARGET_OS_IOS(seems not used) with macros defined
  in TargetConditionals.h to correctly determine platform target.

- This only replaces the macro in urlhook.cpp preventing the use
  of the system function, which is critical to the iOS build.

Change-Id: I4ba40aefe212bf1279a4813d1cdc2d14ddbd4273
Reviewed-by: Kateryna Kostiuk <kateryna.kostiuk@savoirfairelinux.com>