The Go Book (gopl.io) code is stuffed full of such undocumented build-constraint-looking comments. There is nothing about them @ golang.org/pkg/go/build/#hdr-Build_Constraints. Any ideas on what they do; how to use them and such?
It just seems as build directives. You can tweak your build on basis of platform, os, etc. You determine if the files should be included in the build on these criteria.