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>

src/hooks/urlhook.cpp

@@ -29,7 +29,12 @@ int UrlHook::runAction(const std::string &command, const std::string &args)
     const std::string cmd = command + (args.empty() ? "" : " ") +
                             "\"" + args + "\" &";
 
-#if defined(RING_UWP) || defined(TARGET_OS_IOS)
+#if __APPLE__
+  #include "TargetConditionals.h"
+  #if defined(TARGET_IPHONE_SIMULATOR) || defined(TARGET_OS_IPHONE)
+    return 0;
+  #endif
+#elif defined(RING_UWP)
     return 0;
 #else
     return system(cmd.c_str());