I need some of your help.
I searched everywhere on the internet, but I could not find how to create a plugin for Gaim, the predecessor of Pidgin.
I do NOT want to create a Pidgin plugin. I want to create a Gaim plugin, but since Gaim is very old, well, it is hard to find documentation for it.
(PS : If you know how to write a plugin for Gaim, please note I want to make one for Windows - not Linux.)
(PPS : I'm french. Sorry for my bad English.)
You're going to run into a number of issues here...
First off, Gaim hasn't existed for 15 years, and of course we (the Pidgin core team many of who got involved during the Gaim days) aren't going to support it for that reason.
Secondly, building on windows has always been a pain for us as we had to carry all of the dependencies. I imagine most of the links that you might find are all long dead because of the 15 years that have passed since that. That said you might be able to get away with using our win32-dev directory from https://data.imfreedom.org/pidgin/win32-dev.7z but of course that's completely untested and that directory is used to build the Pidgin 2.x.y releases.
Finally, as you've found out, most of the documentation from Gaim has been gone for a very long time. We did set up https://gaim.pidgin.im as a joke which was the last copy of the site we had before the rename, but there's not much there when it comes to development documentation. So your best bet is to look at existing plugins. I still have the source code for guifications1 available at https://keep.imfreedom.org/grim/guifications1/file/default.