Fetch SDL2 in CMake

CMakeLists.txt

@@ -30,14 +30,19 @@ target_include_directories(robomaster
     include
 )
 
-find_package(SDL2)
+include(FetchContent)
+FetchContent_Declare (
+    SDL2
+    GIT_REPOSITORY https://github.com/libsdl-org/SDL
+    GIT_TAG release-2.0.20
+    GIT_SHALLOW TRUE
+)
+FetchContent_MakeAvailable(SDL2)
 
-if(SDL2_FOUND)
 add_executable(robomasterapp
     src/sdl.c
 )
 target_link_libraries(robomasterapp
     robomaster
-	SDL2::SDL2-static
+    SDL2-static
 )
-endif()