对‘avformat_find_stream_info’未定义的引用、to the PKG_CONFIG_PATH environment variable

•  错误

AppDec.o：在函数‘FFmpegDemuxer::FFmpegDemuxer(AVFormatContext*)’中：
AppDec.cpp:(.text._ZN13FFmpegDemuxerC2EP15AVFormatContext[_ZN13FFmpegDemuxerC5EP15AVFormatContext]+0x19e)：
对‘avformat_find_stream_info’未定义的引用      

• 错误 

Package libavcodec was not found in the pkg-config search path.
Perhaps you should add the directory containing `libavcodec.pc'
to the PKG_CONFIG_PATH environment variable      

•  解决办法

增加一个环境变量：

export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:${PKG_CONFIG_PATH}