"configure" script do not find the "mkdirat" function, which is used in the netsys_c.c
Many OSes have the "openat" but have not other "*at" functions. For example, OpenSolaris & DragonFly BSD.
src/netsys/configtests/atfunctions.c checks "openat" only. All used "*at" functions must be checked there (at least linkat, symlinkat, mkfifoat, readlinkat).