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.
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