Since Common.php (which defines get_config()) is loaded after the constants, you can't do this without modifying CI's core files.
My best recommendation to work around this would be to put these definitions in a separate config file (for example, /application/config/app_constants.php), then add that file to the list in $autoload['config'] in /application/config/autoload.php.
If you need the constants defined earlier (before the CI_Controller's constructor is executed), you may need to manually load the new config file from a hook or another file (maybe even /application/config/config.php, though I would generally advise against calling a function or including another file from within a config file, if it's possible to avoid doing so).