Annotation for maybe unused code is missing#45

Hello, I am the author of Nuitka a Python to C11 transpiler, and use macros in it a lot. I only now discovered Hedley when trying to checkout if there was improvements to `likely` and `unlikely` available, and indeed I found this. What I came across going over my include file is this: ``` /* A way to not give warnings about things that are declared, but might not * be used like in-line helper functions in headers or static per module * variables from headers. */ #ifdef __GNUC__ #define NUITKA_MAY_BE_UNUSED __attribute__((__unused__)) #else #define NUITKA_MAY_BE_UNUSED #endif ``` Frequently in code generation, I might declare things that end up being unused and will then be picked up by the C compiler. It is however nothing like this in hedley. As you can see, this would very well need more compiler support, and fall into the category of what hedley does, but I didn't find anything that ressembles it or comes close to it. Do you think this can be added into the scope, or do you think it's nonsense, because normal code doesn't want to have this.