编译ffmpeg例程时，报错：undefined reference

编译ffmpeg例程，报了未定义错误：

main.cpp:(.text+0x3a): undefined reference to avcodec_send_frame(AVCodecContext*, AVFrame const*)'
main.cpp:(.text+0x8c): undefined reference toav_packet_unref(AVPacket*)’
main.cpp:(.text+0x97): undefined reference to avcodec_receive_packet(AVCodecContext*, AVPacket*)'
build/src/main.cpp.o: In functionmain’:
main.cpp:(.text.startup+0x4b): undefined reference to avcodec_register_all()'
main.cpp:(.text.startup+0x53): undefined reference toavcodec_find_encoder_by_name(char const*)’
main.cpp:(.text.startup+0x5d): undefined reference to avcodec_find_encoder(AVCodecID)'
main.cpp:(.text.startup+0x71): undefined reference toavcodec_alloc_context3(AVCodec const*)’
main.cpp:(.text.startup+0x84): undefined reference to av_packet_alloc()'
main.cpp:(.text.startup+0xf9): undefined reference toavcodec_open2(AVCodecContext*, AVCodec const*, AVDictionary**)’
main.cpp:(.text.startup+0x11f): undefined reference to av_frame_alloc()'
main.cpp:(.text.startup+0x154): undefined reference toav_frame_get_buffer(AVFrame*, int)’
main.cpp:(.text.startup+0x182): undefined reference to av_frame_make_writable(AVFrame*)'
main.cpp:(.text.startup+0x2f0): undefined reference toavcodec_free_context(AVCodecContext**)’
main.cpp:(.text.startup+0x2fa): undefined reference to av_frame_free(AVFrame**)'
main.cpp:(.text.startup+0x304): undefined reference toav_packet_free(AVPacket**)’
main.cpp:(.text.startup+0x3aa): undefined reference to `av_opt_set(void*, char const*, char const*, int)’

原因：
ffmpeg是纯C的库，头文件没有做好C++调用的准备
用extern “C”{}套住ffmpeg头文件，用C语言的编译规则来编译ffmpeg代码，就可以了