obstack: Remove ancient NeXTSTEP gcc support conditional This change will ease merging with glibc. The "#if ... __NEXT__" causes a warning with -Wundef which glibc now enables by default. Problem reported by Will Newton in <http://lists.gnu.org/archive/html/bug-gnulib/2014-03/msg00032.html>. glibc <sys/cdefs.h> now uses __extension__ for GCC 2.8 or later, so go with that. * lib/obstack.h (__extension__):