mark unused variable in partable way with no API changes

[shssf]

We have several options to do the same

• attribute((unused)) it is GCC compiler specific
• [[maybe_unused]] but it requires C++17 and brings changes (at least in style) in API line https://en.cppreference.com/w/cpp/language/attributes/maybe_unused

Need cross platform and cross compiler solution with minimum changes in source code (style and lines length)

So, according to the KISS principle I did this PR