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
-    )
-endif()
+add_executable(robomasterapp
+    src/sdl.c
+)
+target_link_libraries(robomasterapp
+    robomaster
+    SDL2-static
+)