TL;DR: undefined variable is dereferenced instead of build matrix configuration
I'm having a heck of a time figuring out how to build CGNS on windows, the documentation appears outdated, so I am trying to emulate your AppVeyor configuration, and I noticed that you probably are not actually testing the build matrix that you intend to test.
Also, does CGNS require the 1.8 branch of HDF5? Does building on Windows require HDF5?
I have Windows support as a requirement for software development/modifaction work that I am doing, as well as exodus support, and currently, the legacy software is using CGNS (and isn't ported to windows).