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, 0 insertions, 2 deletions

diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index 1c91f2e6a3..4238d7930b 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -52,8 +52,6 @@
 
 /* Enable declarations of GNU extensions, since we are compiling them.  */
 #define _GNU_SOURCE	1
-/* And we also need the data for the reentrant functions.  */
-#define _REENTRANT	1
 
 #include <config.h>