Fix limits.h NL_NMAX namespace (bug 19929).

Fix limits.h NL_NMAX namespace (bug 19929).

bits/xopen_lim.h (included by limits.h if __USE_XOPEN) defines
NL_NMAX, but this constant was removed in the 2008 edition of POSIX so
should not be defined in that case.  This patch duly disables that
define for __USE_XOPEN2K8.  It remains enabled for __USE_GNU to avoid
affecting sysconf (_SC_NL_NMAX), the implementation of which uses
"#ifdef NL_NMAX".

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	[BZ #19929]
	* include/bits/xopen_lim.h (NL_NMAX): Do not define if
	[__USE_XOPEN2K8 && !__USE_GNU].
	* conform/Makefile (test-xfail-XOPEN2K8/limits.h/conform): Remove
	variable.