sdl: Allow changing the SDL.h path

demo/sdl_renderer/nuklear_sdl_renderer.h

@@ -11,7 +11,10 @@
 #ifndef NK_SDL_RENDERER_H_
 #define NK_SDL_RENDERER_H_
 
-#include <SDL.h>
+#ifndef NK_SDL_RENDERER_SDL_H
+#define NK_SDL_RENDERER_SDL_H <SDL.h>
+#endif
+#include NK_SDL_RENDERER_SDL_H
 NK_API struct nk_context*   nk_sdl_init(SDL_Window *win, SDL_Renderer *renderer);
 NK_API void                 nk_sdl_font_stash_begin(struct nk_font_atlas **atlas);
 NK_API void                 nk_sdl_font_stash_end(void);