While reading uname.c (in order to recreate it) I found a variable called HOST_OPERATING_SYSTEM (line 371). It's never mentioned in the same file (as Strg+F told me) and not in the dependencies of uname.c, as far as I could see.
Where is the variable initialized and how to access it?
HOST_OPERATING_SYSTEM is defined at build time using autoconf modules defined in gnulib.