add gnucobol (master)

What would be the necessary parts to add this?

It could be useful to have a "floating" GC 3.x and 4.x (think of it like the situation with Python2 and Python3, just that GC4 is a bit like Python3 was in the beginning).

How would that be triggered?
What would be the preferred way to build - likely not SVN or a Git Mirror (or possibly "definitely"?), but a nightly tarball (we build them for 3.x and 4.x on Appveyor and the most active mirror on GitHub provides them as well - both likely with an "artifact retention" (maybe try to upload, but only if the version changed, then download via ftp/rsync?).

As soon as there's more information I can start working on the build.sh (and once this is done, we likely need the compiler config part done as well).

Start by reading build.sh and the compiler configuration referenced in the issue. Review how the AppVeyor nightly tarballs for GnuCOBOL 3.x and 4.x are produced and obtained, then identify the triggering and version-selection requirements. Done means the preferred source and build flow are agreed and both compiler versions can be supported.

Written by the indexing model from the issue text.