Unable to compile FFmpeg on Ubuntu 20.04

In the ffbuild directory (path : ffmpeg_sources/ffmpeg/ffbuild/), inspecting the config.log gives more info on the error message :

/usr/bin/ld: cannot find -lunistring
collect2: error: ld returned 1 exit status
ERROR: gnutls not found using pkg-config

With help from ffmpeg's IRC channel, installing the libunistring-dev package fixed the problem.

sudo apt-get install libunistring-dev