我正在尝试安装pHash，但是当我执行./configure此消息时返回：
...
*** Configuring video Hash ***
checking whether FFmpeg is present... checking for avcodec_alloc_frame in -lavcodec... no
configure: error:
*** libavcodec not found.
You need FFmpeg. Get it at <http://ffmpeg.org/>
但是 FFMPEG 已正确安装，实际上执行ffmpeg它会返回：
ffmpeg version 3.0.2 Copyright (c) 2000-2016 the FFmpeg developers
built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.1)
configuration:
libavutil 55. 17.103 / 55. 17.103
libavcodec 57. 24.102 / 57. 24.102
libavformat 57. 25.100 / 57. 25.100
libavdevice 57. 0.101 / 57. 0.101
libavfilter 6. 31.100 / 6. 31.100
libswscale 4. 0.100 / 4. 0.100
libswresample 2. 0.101 / 2. 0.101
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...
Use -h to get full help or, even better, run 'man ffmpeg'
为什么会发生这种情况？