Wow don't be so vindicative both of you

No trouble if you copy custom files into include and lib folders of the compiler. We just have to know where the compiler is looking for those files during compilation.
But you are right, it's not very clean to do like that.
In fact I noticed that Code::blocks is delivred with a very old version of GCC (GCC8.1 !!). And with this version of GCC, we need to precise the include and lib path in the search directory otherwise it doesn't work (so the tutorials are right). But with a more recent version of GCC (I use GCC10.3) it is not necessary.
Thank you for all your replies.