You make an excellent point. Perhaps more so than with other languages, getting to grips with making C actually useful relies a lot on quite extensive knowledge of the build environment.
You also have to know about how computers work, how they represent numbers, how and where they store memory, how computers process information, you have to know about interrupts low level os stuff, networking protocols, physical hardware limitations, debugging low level byzantine failures. You have to know a lot more than just C syntax. I haven't even mentioned computer science, best practices, working with others, architecture and UI topics.
It's not only the build environment. Mastering C goes way beyond the superficial level of just learning the language and the libraries. Being a very thin abstraction over the machine, you will inevitably encounter some quirks which can only be explained from the perspective of a compiler writer or a hardware designer. Also, POSIX is not just another library, it's the interface between your program and the operating system. Knowing what to call and how to call it ultimately requires a good foundation in OS theory.
