Prefer the __gnu_hurd__ macro over __GNU__. * lib/get_ppid_of.c: Replace use of __GNU__ with __gnu_hurd__. * lib/get_progname_of.c: Likewise. * lib/sigsegv.c: Likewise. * lib/sigsegv.in.h: Likewise. * lib/vma-iter.c: Likewise. * lib/vma-iter.h: Likewise. * tests/test-ptsname.c: Likewise. * tests/test-ptsname_r.c: Likewise. * tests/test-sigsegv-catch-stackoverflow1.c: Likewise. * tests/test-sigsegv-catch-stackoverflow2.c: Likewise. * lib/getloadavg.c: Likewise. Adjust conditions and comments to account for the fact that NeXT defines __GNU__ and not __gnu_hurd__.