_REENTRANT should be unused

["Joseph S. Myers" <joseph at codesourcery dot com>]

The feature test macro _REENTRANT has somewhat misleading or out of date 
documentation:

	@defvr Macro _REENTRANT
	@defvrx Macro _THREAD_SAFE
	If you define one of these macros, reentrant versions of several functions get
	declared.  Some of the functions are specified in POSIX.1c but many others
	are only available on a few other systems or are unique to GNU libc.
	The problem is the delay in the standardization of the thread safe C library
	interface.

	Unlike on some other systems, no special version of the C library must be
	used for linking.  There is only one version but while compiling this
	it must have been specified to compile as thread safe.
	@end defvr

In fact, the *only* uses of __USE_REENTRANT which <features.h> defines 
given _REENTRANT or _THREAD_SAFE are to condition the declaration of 
getlogin_r in posix/unistd.h

#if defined __USE_REENTRANT || defined __USE_UNIX98

and similarly for the checking version in posix/bits/unistd.h.

__USE_UNIX98 should be __USE_POSIX199506 in this case, as getlogin_r is 
included in POSIX.1:1996.  __USE_POSIX199506 is defined by default when 
users don't use any feature test macros.  Given this, the utility of 
_REENTRANT to control the declaration of a single function in the special 
case where the user has disabled declarations of POSIX.1:1996 functions 
seems very doubtful to me.  Perhaps the macro, its documentation and the 
#define of it in include/libc-symbols.h should be removed?

-- 
Joseph S. Myers
joseph@codesourcery.com