Update ios.toolchain.cmake

Added fix if upgrading Xcode versions on consequent builds

Author: leetal

ios.toolchain.cmake

@@ -182,20 +182,22 @@ endif()
 message(STATUS "Configuring iOS build for platform: ${IOS_PLATFORM}, "
   "architecture(s): ${IOS_ARCH}")
 # If user did not specify the SDK root to use, then query xcodebuild for it.
-if (NOT CMAKE_OSX_SYSROOT)
+if (NOT EXISTS ${CMAKE_OSX_SYSROOT})
   execute_process(COMMAND xcodebuild -version -sdk ${XCODE_IOS_PLATFORM} Path
     OUTPUT_VARIABLE CMAKE_OSX_SYSROOT
     ERROR_QUIET
     OUTPUT_STRIP_TRAILING_WHITESPACE)
-  message(STATUS "Using SDK: ${CMAKE_OSX_SYSROOT} for platform: ${IOS_PLATFORM}")
-endif()
-if (NOT EXISTS ${CMAKE_OSX_SYSROOT})
-  message(SEND_ERROR "Please make sure that Xcode is installed and that the toolchain"
-    "is pointing to the correct path. Please run:"
-    "sudo xcode-select -s /Applications/Xcode.app/Contents/Developer"
-    "and see if that fixes the problem for you.")
-  message(FATAL_ERROR "Invalid CMAKE_OSX_SYSROOT: ${CMAKE_OSX_SYSROOT} "
-    "does not exist.")
+    
+    if (NOT EXISTS ${CMAKE_OSX_SYSROOT})
+      message(SEND_ERROR "Please make sure that Xcode is installed and that the toolchain"
+      "is pointing to the correct path. Please run:"
+      "sudo xcode-select -s /Applications/Xcode.app/Contents/Developer"
+      "and see if that fixes the problem for you.")
+      message(FATAL_ERROR "Invalid CMAKE_OSX_SYSROOT: ${CMAKE_OSX_SYSROOT} "
+      "does not exist.")
+    else()
+       message(STATUS "Using SDK: ${CMAKE_OSX_SYSROOT} for platform: ${IOS_PLATFORM}")
+    endif()
 endif()
 # Specify minimum version of deployment target.
 if (NOT DEFINED IOS_DEPLOYMENT_TARGET)