cmake: create SDL3::headers for include path + no exported CMake variables

cmake/test/inc_sdl_slash.c

@@ -0,0 +1,8 @@
+#include "SDL3/SDL.h"
+#include "SDL3/SDL_main.h"
+
+void inc_sdl_slash(void) {
+    SDL_SetMainReady();
+    SDL_Init(SDL_INIT_EVERYTHING);
+    SDL_Quit();
+}