Setup github actions for builds

[davidrg]

This will likely depend on getting modern versions of Visual C++ working (#5 ) at a minimum (I doubt Microsoft puts antique compilers on their build agents). Would be good if some minimal automated tests could be performed too.

When OpenWatcom (#7 ) and OS/2 (#8 ) are sorted it should be possible to cross-compile for OS/2 from the windows or linux build agents.