Set the priority for the current thread.
int SDL_SetThreadPriority(SDL_ThreadPriority priority);
the SDL_ThreadPriority to set
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
Note that some platforms will not let you alter the priority (or at least, promote the thread to a higher priority) at all, and some require you to be an administrator account. Be prepared for this to fail.
This function is available since SDL 2.0.0.