Create custom #warning flags

I'm building a commercial app, and we are using some GPL code to help us along. How can I add `#warning` or `#error` statements so that when the code is built for debug, it warns, but when we build for release it throws errors? I can do: ``` #warning this code is released under a CCL licensing scheme, see Source_Code_License.rtf #warning this code is not LGPL-compliant #warning this code was copied verbatim from a GP Licensed file ``` at the beginning of files, but can I do better? Is there a better way of tagging a file if it's included? I'm using Objective-C++ with gcc or clang.