Although FFmpeg is installed, OBS currently fails to build on debian
(tested on Debian Jessie 8.6). It looks for libavcodec.a in
/usr/local/lib and not /usr/lib.
Closesjp9000/obs-studio#661
This allows people to use the base directory of cross-compiled
dependencies (typically our dependencies.zip file), rather than always
having to specify the actual include directory.