check if policy exists before enabling it

CMakeLists.txt

@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 3.0.0)
-cmake_policy(SET CMP0069 NEW)
+if(POLICY CMP0069)
+  cmake_policy(SET CMP0069 NEW)
+endif()
 
 project(HDF4CPP LANGUAGES CXX)