> [ERROR]  Your file system in '/mnt/d/embedded_linux_course/toolchain_understandin/crosstool-ng/.build' is *not* case-sensitive!

Isn't this your problem?
Your filesystem isn't case sensitive and building certain stuff (e.g. Linux and maybe also a Linux toolchain) requires a case sensitive filesystem because some Linux headers have the same name distinguished only by case (e.g. Foo.h and foo.h).