If this is about library inclusion and building, that's generally pretty specific to Linux users I think. Setup the feature to work if the library is present and not to break if it isn't. Then let Linux users decide at build time.

If this is just about the feature at all for general users, it should be implemented into the new Options menu or a game_settings.tbl flag while taking a deeper look into the idea in #3934 for user settings. I'm generally against adding more commandline flags, though admittedly this one actually seems to fit there slightly more than a lot of the junk we currently have... That's my 2c not that my opinion matters much on the issue.

If going commandline, many of the graphics flags are opt-out rather than opt-in. Maybe follow the same trend.

Maybe a short background explaination:
Most Linux distros are 'binary', meaning the packages they install are pre-compiled and the package maintainer decides which features to include. Same for the AppImage. Then there are 'source' based distros (they are a minority) where the end-user decides which features to include. They are presented with a list of features to include/exclude at build time. If we disable discord-rpc it means don't build the discord library and remove all calls to that library.
For the 'binary' distros or Mac OS / Windows which have the feature compiled in by default, it would certainly be nice to have a cmd line option to opt-out sharing this info online.