No, I have never used SDL.
It sounds to me like you need to speak to the SDL people, not here.
The 'normal' model is you would build something like sdl in a dir inside you own space, using the source tree and running make from the right place. That way, the make file knows where everything it needs is. Once built correctly, you may then install it into places like /use/include.
If the include files have got to /usr/include, then you must have already compiled (successfully) sdl and now want to compile your own app, dependent on the sdl libraries. ??
But, as I said, never built or used SDL.
Davo