Using a newer version gcc and gpp

Sometimes you might want to build on an old base system (Ubuntu 12.04) for example, but need a newer version of GCC or G++. We can do this by adding the following repository to your AppImage.yml file and the version of GCC/G++ you want to your build-requires.

base: 12.04

repositories:
    - ppa:ubuntu-toolchain-r/test

build-requires:
    - gcc-5
    - g++-5