Add a warning

Author: jan-cerny

CMakeLists.txt

@@ -466,6 +466,8 @@ include_directories(
 # Run "cmake --help-policy CMP0063" for policy details
 if (POLICY CMP0063)
 	cmake_policy(SET CMP0063 NEW)
+else()
+	message(WARNING "It is not possible to correctly set symbol visibility in object files with your version of CMake. We recommend using CMake 3.3 or newer.")
 endif()
 
 function(set_oscap_generic_properties TARGET_OBJECT)