build ffmpeg scripts and other third party libary.
- we use ffmpeg release/4.4 base branch for hevc, new feature in feature/support_hevc_base_rc4.4
- new ffmpeg has support hevc in release/6.1, details see: FFmpeg直播能力更新计划与新版本发布
- step1: if need x265, download source code from https://www.x265.org/downloads/ by yourself. then save x265_v3.3.tar.gz to 3rd dir.
- step2: run scripts/build_ffmpeg.sh and get ffmpeg release in install dir.