cmake: create SDL2::SDL2main target in Apple framework official release

Xcode/SDL/pkg-support/resources/CMake/sdl2-config.cmake

@@ -58,7 +58,10 @@ if(NOT TARGET SDL2::SDL2)
             COMPATIBLE_INTERFACE_BOOL "SDL2_SHARED"
             INTERFACE_SDL2_SHARED "ON"
     )
+    set(SDL2_SDL2_FOUND TRUE)
 endif()
-set(SDL2_SDL2_FOUND)
+
+add_library(SDL2::SDL2main INTERFACE IMPORTED)
+set(SDL2_SDL2main_FOUND TRUE)
 
 check_required_components(SDL2)