but my library should be only in C.
Why?
The reason I wanted to create the library is that I have also read that void functions need to be put in a separate library, especially if they should take custom data types as inputs.
This is crap. Functions of any type, taking any kind of values, can be in the sketch with the loop() and setup() functions. A library is to make code reusable.
Meanwhile, if somebody has managed to get the code from wikipedia to compile -- first of all, congrats! -- please share
You could go the other way, and post your code (you admit that you read that you were supposed to) and post the error messages.