could not install package(s) gcc-6 - Debian buster

[philBrown]

gcc-6 is required to compile "GBA emu - gpSP port for libretro"

On a Debian Buster image of OSMC, gcc-6 is not in the standard repositories but it can be found in oldstable (Stretch). Add the oldstable apt source before trying to install Retropie

sudo vi /etc/apt/sources.list

add the following two lines

deb http://ftp.debian.org/debian/ oldstable main contrib non-free
deb-src http://ftp.debian.org/debian/ oldstable main contrib non-free

then update

sudo apt update