It's not completely clear to me what the issue is now.
It is to select with ethernet to chose for the compiler without have to edit a #define inside .h file.
because the libraries concern could not have only .h file cause of size and/or complexity, For translation unit, i did not see a solution.
For the idea of only include client or server, for one of my libraries, that will be possible I think.
But for the websocket I try to use, it need more of ethernet like tcp and socket class.