Add GOG.com build system, GOG variants of FTL & Reus, and new GOG TIS-100 package

Added gog-build-system, which is mostly like binary-build-system but with no install-plan because every game installs the same. FTL and Reus were my test cases because they were in my GOG library and already packaged here.

TIS-100 is the game I actually wanted to play, and is possibly the first Unity3D game to get packaged. It was tricky to get working because some of its patchelf dependencies aren't listed by running ldd on the binaries. I only mention it because it may be useful to look at for the next Unity3D game.

gog-fetch method is still missing. I thought it would be a simple matter of invoking lgogdownloader, but its authentication is stateful. It logs in interactively on first run with your username & password, and also possibly 2FA and captcha, and then stores secrets for subsequent runs in your config file and a cookie file. I don't really know how to tame it for Guix.