When XCode changes for a rebuild if needed.

xcode.sh

@@ -1,8 +1,25 @@
 package: Xcode
 version: "1.0"
-system_requirement_missing: |
-  Please make sure you install Xcode and the command line tools using xcode-select --install
-system_requirement: "(osx.*)"
-system_requirement_check: |
+prefer_system: ".*"
+prefer_system_check: |
+  # In case we are not on Darwin, this should not be included
+  case `uname -o` in
+    Darwin) ;;
+    *) echo "alibuild_system_replace: unsupported" ;;
+  esac
   xcode-select -p && echo '#include <AvailabilityMacros.h>' | clang++ -x c++ - -c -o /dev/null
+  XCODE_VERSION="$(xcode-select -v | sed -e 's/[^0-9]//g')"
+  echo "alibuild_system_replace: ${XCODE_VERSION}"
+prefer_system_replacement_specs:
+  unsupported:
+    version: "unsupported"
+    recipe: |
+      echo "XCode not supported on $(uname -o). Please check your dependencies."
+      exit 1
+  "[0-9]+":
+    version: "%(key)s"
+    recipe: |
+      exit 0
 ---
+echo "Please make sure you install Xcode and the command line tools using xcode-select --install."
+exit 1