I'm inclined to say that it is not a GCC/g++ problem, but a wxWidgets problem. They use long recursive macro chains that finally end up expanding to function attributes, and some of them are duplicate or in places where they shouldn't be, and the compiler tells you that it doesn't like it.
The good thing about it is that with this evil, ugly preprocessor hack, they somehow manage to compile their code on nearly every compiler from 15 year old ones to present ones. The bad thing is that you get so much noise, which effectively makes compiler warnings useless.
Compiling Code::Blocks with gcc 4.2 gives >2000 warnings from including the wxWidgets headers, compiling wxWidgets gives so many warnings that you cannot even count them (on the order of hundred thousands or millions).
Wwolf is right insofar as we can indeed not do much about it (nothing at all, actually). Just ignore it, or use the compiler option to turn off attribute warnings (this won't affect any other warnings).