SDL_Texture - Incomplete type
There's nothing wrong with your install. SDL_Texture is an opaque type by design (that is, designed to only be operated on by SDL2 internally), you can "pass it around" as a pointer, but you can't access the internals (or create a SDL_Texture yourself, for example by doing a malloc, because you don't know the size of the structure). If you stick to
SDL_Texture *blah;pointers and pass them around to the SDL2 functions you should be fine.
SDL_sysrender.h is an internal header which, as you mentioned, actually defines SDL_Texture for internal consumption of the library.