iOS/Android: cmake message if SDL not downloaded

external/CMakeLists.txt

@@ -48,7 +48,14 @@ endif()
 if (HELLOIMGUI_USE_SDL_OPENGL3)
     # On desktop (linux, windows, osx) sdl is provided via vcpkg
     # On iOS, the build is done externally via tools/ios/sdl_compile_ios.sh
+
     if (ANDROID OR IOS)
+        # if the folder SDL is not present, it means that the user did not download the submodule
+        # we download it for him
+        if(NOT IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/SDL)
+            message(FATAL_ERROR "Please download SDL by running ${HELLOIMGUI_BASEPATH}/tools/sdl_download.sh")
+        endif()
+
         add_subdirectory(SDL)
     endif()
 endif()