cmake: support some policies

CMakeLists.txt

 cmake_minimum_required (VERSION 3.1)
+if(POLICY CMP0065)
+  cmake_policy(SET CMP0065 NEW)
+endif()
+if(POLICY CMP0069)
+  cmake_policy(SET CMP0069 NEW)
+endif()
+if(POLICY CMP0073)
+  cmake_policy(SET CMP0073 NEW)
+endif()
+if(POLICY CMP0074)
+  cmake_policy(SET CMP0074 NEW)
+endif()
 project (opendht)
 
 include(CMakePackageConfigHelpers)