 nonposix.h: define _GNU_SOURCE for Hurd, remove __GLIBC__ test
 nonposix.h: define _GNU_SOURCE for Hurd, remove __GLIBC__ test

__GLIBC__ and similar ftms are not defined in the compiler, but
in features.h, which isn't POSIX so cannot be included, and
standard headers can't be included either because nonposix.h
defines ftms that must exist *prior to* std headers inclusion.

The kernel ftms, however, are defined by the compiler, so we can
use them.

skarnet/skalibs,skarnet/skalibs