Support the GOG version of Re-mars-tered

[Moneyl]

RSL should support the GOG version of re-mars-tered. It currently doesn't do anything with steam specific code so adding support would likely only be a matter of adding a compilation option for using steam or gog function and value offsets, and finding those offsets for GOG. That part shouldn't be difficult, just time consuming.

The part I'm not sure about is how to keep automatic builds from doubling in time. My current thought is to just have a compilation flag for whether Steam or GOG offsets should be used. The problem with that is that we'd then have to build it twice unless there's a way to build two compile configs at once in appveyor. Once for GOG, once for Steam.