Oh by the way, SDL_BUTTON_X1/2 need SDL2 2.0.4 or newer to work correctly on Linux, you're shipping 2.0.3 so you may wanna upgrade that (2.0.4 also has lots of other bugfixes, so upgrading is a good idea even if you choose not to support these buttons).
I tested SlayerShock with 2.0.4 and the buttons didn't work with that either, so I guess it's not supported by your code yet.