Fix building with CMake versions below 3.24

There's also no need to explicitly set CMP0015 to NEW as that's included
when setting the minimum required CMake version.

CMakeLists.txt

@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 3.8)
-cmake_policy(SET CMP0015 NEW)
-cmake_policy(SET CMP0135 NEW)
+if(POLICY CMP0135)
+    cmake_policy(SET CMP0135 NEW)
+endif()
 project(libloot)
 include(ExternalProject)