[PATCH 2/2] sys/cdefs.h: fix for use __restrict in C++

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Newlib shares large parts of <sys/cdefs.h> with FreeBSD and received
this bug report:

https://sourceware.org/pipermail/newlib/2023/020400.html

As an extension, GCC and clang offer C99-style restricted pointers in
C++ mode:
https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html

We notice that this extension is broken when including newlib headers:
restricted pointers are treated as ordinary pointers.

We traced this to the following section of
newlib/libc/include/sys/cdefs.h:

  /*
   * GCC 2.95 provides `__restrict' as an extension to C90 to support the
   * C99-specific `restrict' type qualifier.  We happen to use `__restrict' as
   * a way to define the `restrict' type qualifier without disturbing older
   * software that is unaware of C99 keywords.
   */
  #if !(__GNUC__ == 2 && __GNUC_MINOR__ == 95)
  #if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901
  #define	__restrict
  #else
  #define	__restrict	restrict
  #endif
  #endif

While the GCC __restrict extension was indeed introduced in GCC 2.95, it
is not limited to this version; the extension is also not limited to
C90:
https://gcc.gnu.org/gcc-2.95/c++features.html

Rewrite the logic in the header so that __restrict is kept alone when
available.

PR:	272723
MFC after:	1 week
---
 newlib/libc/include/sys/cdefs.h | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/newlib/libc/include/sys/cdefs.h b/newlib/libc/include/sys/cdefs.h
index 808498c50b..cc1a8a1ccb 100644
--- a/newlib/libc/include/sys/cdefs.h
+++ b/newlib/libc/include/sys/cdefs.h
@@ -412,17 +412,15 @@
 #endif
 
 /*
- * GCC 2.95 provides `__restrict' as an extension to C90 to support the
- * C99-specific `restrict' type qualifier.  We happen to use `__restrict' as
- * a way to define the `restrict' type qualifier without disturbing older
- * software that is unaware of C99 keywords.
+ * We use `__restrict' as a way to define the `restrict' type qualifier
+ * without disturbing older software that is unaware of C99 keywords.
+ * GCC also provides `__restrict' as an extension to support C99-style
+ * restricted pointers in other language modes.
  */
-#if !(__GNUC__ == 2 && __GNUC_MINOR__ == 95)
-#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901
-#define	__restrict
-#else
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901
 #define	__restrict	restrict
-#endif
+#elif !__GNUC_PREREQ__(2, 95)
+#define	__restrict
 #endif
 
 /*
-- 
2.35.3