I didn't look at the code (sorry), but those files that depends on GPL code (either by calling GPL functions that are not described by a standard, or using data structures defined in GPL code or using global variables there defined), can only be licensed as GPL.
You cannot build a (say) MIT derivative of a GPL library, for example. What you can do (I imagine) is to double license your new files as MIT and GPL, so that if all the GPL code is replaced with MIT code that do not violates the original copyright (that is, the whole application is rewritten from scratch IN A TOTALLY DIFFERENT WAY) the future contributor can decide to terminate their own GPL license on those files and leverage only the MIT one during their migration to the new codebase. As of today, if those file depend on GPL code, the authors can't distribute them under a different license. Actually they might have terminated their right to create a derived work by doing so. This wouldn't affect (AFAIK) people who receive such GPL derivative and threat it as GPL derivative, though. So adding the missing header WHERE the GPL dependency actually exists (that can only be known by carefully analyzing each file source), might be a simple and effective fix to settle the matter. Giacomo
