Add feature test macro _ISOC2Y_SOURCE

This patch starts preparation for C2Y support in glibc headers by
adding a feature test macro _ISOC2Y_SOURCE and corresponding
__GLIBC_USE (ISOC2Y). (I mostly copied the work of Joseph Myers
for C2X). As with other such macros, C2Y features are also
enabled by compiling for a standard newer than C23, or by using
_GNU_SOURCE.

This patch does not itself enable anything new in the headers for C2Y;
that is to be done in followup patches. (For example an implementation
of WG14 N3349.)

Once C2Y becomes an actual standard we'll presumably move to using the
actual year in the feature test macro and __GLIBC_USE, with some
period when both macro spellings are accepted, as was done with
_ISOC2X_SOURCE.

Tested for x86_64.

Signed-off-by: Lenard Mollenkopf <glibc@lenardmollenkopf.de>

NEWS

@@ -31,6 +31,13 @@ Major new features:
 * The iconv program now supports converting files in place.  The program
   automatically uses a temporary file if required.
 
+* The GNU C Library now supports a feature test macro _ISOC2Y_SOURCE to
+  enable features from the draft ISO C2Y standard.  Only some features from
+  this draft standard are supported by the GNU C Library, and as the draft
+  is under active development, the set of features enabled by this macro is
+  liable to change.  Features from C2Y are also enabled by _GNU_SOURCE, or
+  by compiling with "gcc -std=gnu2y".
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The big-endian ARC port (arceb-linux-gnu) has been removed.

include/features.h

@@ -26,6 +26,7 @@
    _ISOC11_SOURCE	Extensions to ISO C99 from ISO C11.
    _ISOC23_SOURCE	Extensions to ISO C99 from ISO C23.
    _ISOC2X_SOURCE	Old name for _ISOC23_SOURCE.
+   _ISOC2Y_SOURCE	Extensions to ISO C23 from ISO C2Y.
    __STDC_WANT_LIB_EXT2__
 			Extensions to ISO C99 from TR 27431-2:2010.
    __STDC_WANT_IEC_60559_BFP_EXT__
@@ -150,6 +151,7 @@
 #undef	__USE_FORTIFY_LEVEL
 #undef	__KERNEL_STRICT_NAMES
 #undef	__GLIBC_USE_ISOC23
+#undef	__GLIBC_USE_ISOC2Y
 #undef	__GLIBC_USE_DEPRECATED_GETS
 #undef	__GLIBC_USE_DEPRECATED_SCANF
 #undef	__GLIBC_USE_C23_STRTOL
@@ -216,6 +218,8 @@
 # define _ISOC11_SOURCE	1
 # undef  _ISOC23_SOURCE
 # define _ISOC23_SOURCE	1
+# undef  _ISOC2Y_SOURCE
+# define _ISOC2Y_SOURCE	1
 # undef  _POSIX_SOURCE
 # define _POSIX_SOURCE	1
 # undef  _POSIX_C_SOURCE
@@ -239,15 +243,23 @@
 #if (defined _DEFAULT_SOURCE					\
      || (!defined __STRICT_ANSI__				\
 	 && !defined _ISOC99_SOURCE && !defined _ISOC11_SOURCE	\
-	 && !defined _ISOC23_SOURCE				\
+	 && !defined _ISOC23_SOURCE && !defined _ISOC2Y_SOURCE	\
 	 && !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE	\
 	 && !defined _XOPEN_SOURCE))
 # undef  _DEFAULT_SOURCE
 # define _DEFAULT_SOURCE	1
 #endif
 
+/* This is to enable the ISO C2Y extension.  */
+#if (defined _ISOC2Y_SOURCE \
+     || (defined __STDC_VERSION__ && __STDC_VERSION__ > 202311L))
+# define __GLIBC_USE_ISOC2Y	1
+#else
+# define __GLIBC_USE_ISOC2Y	0
+#endif
+
 /* This is to enable the ISO C23 extension.  */
-#if (defined _ISOC23_SOURCE \
+#if (defined _ISOC23_SOURCE || defined _ISOC2Y_SOURCE \
      || (defined __STDC_VERSION__ && __STDC_VERSION__ > 201710L))
 # define __GLIBC_USE_ISOC23	1
 #else
@@ -256,20 +268,21 @@
 
 /* This is to enable the ISO C11 extension.  */
 #if (defined _ISOC11_SOURCE || defined _ISOC23_SOURCE	\
+     || defined _ISOC2Y_SOURCE				\
      || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L))
 # define __USE_ISOC11	1
 #endif
 
 /* This is to enable the ISO C99 extension.  */
 #if (defined _ISOC99_SOURCE || defined _ISOC11_SOURCE			\
-     || defined _ISOC23_SOURCE						\
+     || defined _ISOC23_SOURCE || defined _ISOC2Y_SOURCE		\
      || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L))
 # define __USE_ISOC99	1
 #endif
 
 /* This is to enable the ISO C90 Amendment 1:1995 extension.  */
 #if (defined _ISOC99_SOURCE || defined _ISOC11_SOURCE			\
-     || defined _ISOC23_SOURCE						\
+     || defined _ISOC23_SOURCE || defined _ISOC2Y_SOURCE		\
      || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199409L))
 # define __USE_ISOC95	1
 #endif

manual/creature.texi

@@ -233,6 +233,13 @@ Only some features from this draft standard are supported by
 @theglibc{}.  The older name @code{_ISOC2X_SOURCE} is also supported.
 @end defvr
 
+@defvr Macro _ISOC2Y_SOURCE
+@standards{C2Y, (none)}
+If this macro is defined, ISO C2Y extensions to ISO C23 are included.
+Only some features from this draft standard are supported by
+@theglibc{}.
+@end defvr
+
 @defvr Macro __STDC_WANT_LIB_EXT2__
 @standards{ISO, (none)}
 If you define this macro to the value @code{1}, features from ISO/IEC