GenerateExportHeader: Port to use message(DEPRECATION)

Modules/GenerateExportHeader.cmake

@@ -337,15 +337,7 @@ endfunction()
 
 function(add_compiler_export_flags)
   if(NOT CMAKE_MINIMUM_REQUIRED_VERSION VERSION_LESS 2.8.12)
-    if(CMAKE_WARN_DEPRECATED)
-      set(messageType WARNING)
-    endif()
-    if(CMAKE_ERROR_DEPRECATED)
-      set(messageType FATAL_ERROR)
-    endif()
-    if(messageType)
-      message(${messageType} "The add_compiler_export_flags function is obsolete. Use the CXX_VISIBILITY_PRESET and VISIBILITY_INLINES_HIDDEN target properties instead.")
-    endif()
+    message(DEPRECATION "The add_compiler_export_flags function is obsolete. Use the CXX_VISIBILITY_PRESET and VISIBILITY_INLINES_HIDDEN target properties instead.")
   endif()
 
   _test_compiler_hidden_visibility()