set(CMAKE_REQUIRED_LIBRARIES ${FFMPEG_LIBRARIES})
set(CMAKE_REQUIRED_INCLUDES ${FFMPEG_LIBAVFORMAT_INCLUDE_DIRS})
check_symbol_exists(avformat_open_input
set(CMAKE_REQUIRED_LIBRARIES ${FFMPEG_LIBRARIES})
set(CMAKE_REQUIRED_INCLUDES ${FFMPEG_LIBAVFORMAT_INCLUDE_DIRS})
check_symbol_exists(avformat_open_input