Make _REENTRANT and _THREAD_SAFE aliases for _POSIX_C_SOURCE=199506L.

For many years, the only effect of these macros has been to make
unistd.h declare getlogin_r.  _POSIX_C_SOURCE >= 199506L also causes
this function to be declared.  However, people who don't carefully
read all the headers might be confused into thinking they need to
define _REENTRANT for any threaded code (as was indeed the case a long
time ago).

Therefore, remove __USE_REENTRANT, and make _REENTRANT and _THREAD_SAFE
into synonyms for _POSIX_C_SOURCE=199506L.  This will only affect
programs that don't select a higher conformance level some other way.
For instance, -std=c89 -D_REENTRANT will see a change in visible
declarations, but -std=c99 -D_POSIX_C_SOURCE=200809L -D_REENTRANT won't,
and -D_REENTRANT all by itself also won't, because _DEFAULT_SOURCE
implies _POSIX_C_SOURCE > 199506.

	* include/features.h: Remove __USE_REENTRANT.  Treat _REENTRANT
	and _THREAD_SAFE the same as _POSIX_C_SOURCE=199506L, if a higher
	POSIX conformance level has not been selected by other macros.
	* NEWS, manual/creature.texi: Document this change.

	* posix/unistd.h, posix/bits/unistd.h: Don't check __USE_REENTRANT.
	* include/libc-symbols.h: Don't define _REENTRANT.
	* scripts/check-installed-headers.sh: Don't undefine _REENTRANT.

1 files changed, 1 insertions, 1 deletions

diff --git a/posix/bits/unistd.h b/posix/bits/unistd.h
index 0105f04f20..4a5f42e120 100644
--- a/posix/bits/unistd.h
+++ b/posix/bits/unistd.h
@@ -302,7 +302,7 @@ __NTH (ttyname_r (int __fd, char *__buf, size_t __buflen))
 }
 
 
-#if defined __USE_REENTRANT || defined __USE_POSIX199506
+#ifdef __USE_POSIX199506
 extern int __getlogin_r_chk (char *__buf, size_t __buflen, size_t __nreal)
      __nonnull ((1));
 extern int __REDIRECT (__getlogin_r_alias, (char *__buf, size_t __buflen),