Build scripts are not required for open source. Usually they are provided, because nobody actually wants to maintain them separately, but they are not actually part of the project itself. Often times it's just a few gnu scripts, sometimes there's parts of it missing (because they're reused from another project, or they have secrets in them that the maintainer can't be bothered to remove, or other reasons), and rarely the build script is an entire project itself, and even more rarely there's nothing there at all except a single file of source code that can't be built alone (I've seen this in particular on several old golang projects, and it's incredibly annoying).