./configure can't find libavcodec

[MartinNL]

When running ./configure in the /etc/vlc-0.8.6b directory I get the following error:

checking for ffmpeg-config... no
checking for FFMPEG... 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
No package 'libavcodec' found
no
checking ffmpeg/avcodec.h usability... no
checking ffmpeg/avcodec.h presence... no
checking for ffmpeg/avcodec.h... no
configure: error: Missing header file ffmpeg/avcodec.h.

But I installed ffmpeg without any error messages into /etc/ffmpeg-20041113.

How can I add this directory to the PKG_CONFIG_PATH environment variable ???