Konstanty
Add CI via GitHub Actions
This adds some basic CI builds via GitHub Actions.
CMake
This produces a build matrix [Linux, Windows, MacOS] x [Debug, Release] and builds libmodplug with CMake with the default detected compiler in the respective GitHub Actions environment (currently: GCC 9, Visual Studio 2019 and Apple Clang 13).
Autotools
This produces a build matrix [Linux] x [Debug, Release] and build libmodplug with Autotools with the default detected compiler in the Ubuntu GitHub Actions environment (currently: GCC 9)
An exemplary run can be seen here: https://github.com/Croydon/libmodplug/actions/runs/1772652809
Later on, this could be extended to cover more compilers, compiler versions and build systems, but it is a start to get some CI going.
If this is merged, GitHub will run this CI for every future push and pull request, no further configuration is required whatsoever.
AFAICS, cmake support here is still not fully mature (e.g. non-identical library output names for mingw and mac compared to autotools, maybe other issues as well), so, if it were me I'd use autotools for this.
autotools could be added in a separate pull request as well, but I don't think that this is a reason against adding CMake coverage