Testing for older GCC versions?

[awvwgk]

It seems that we can only test for GCC 7 and newer with our available CI runners. However, this project doesn't use any features not available in GCC 5 and should therefore in principle be compatible, yet we have no way for testing this.

While GCC 8 and older are already officially dropped from support upstream, there are probably still users for those compilers. Since it is going to become more difficult to test those versions, providing support on best effort basis is likely the only thing we can offer. If support for older GCC versions breaks, it will be dropped.