Improve port of AC_C_RESTRICT to Oracle C++

Problem reported by Christian Biesinger in:
https://lists.gnu.org/r/bug-gnulib/2019-12/msg00159.html
* lib/autoconf/c.m4 (AC_C_RESTRICT): Port better to
Oracle Developer Studio C++ 12.5 or later.
This patch is adapted from Gnulib.

1 files changed, 7 insertions, 6 deletions

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 601c2df4..14240460 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -2031,12 +2031,13 @@ AC_DEFUN([AC_C_RESTRICT],
    nothing if this is not supported.  Do not define if restrict is
    supported only directly.  */
 #undef restrict
-/* Work around a bug in Sun C++ 5.13: it does not support _Restrict or
-   __restrict__, even though the corresponding Sun C compiler ends up with
-   "#define restrict _Restrict" or "#define restrict __restrict__".
-   Perhaps some future version of Sun C++ will work with restrict;
-   if so, hopefully it defines __RESTRICT like Sun C does.  */
-#if defined __SUNPRO_CC && !defined __RESTRICT
+/* Work around a bug in older versions of Sun C++, which did not
+   #define __restrict__ or support _Restrict or __restrict__
+   even though the corresponding Sun C compiler ended up with
+   "#define restrict _Restrict" or "#define restrict __restrict__"
+   in the previous line.  This workaround can be removed once
+   we assume Oracle Developer Studio 12.5 (2016) or later.  */
+#if defined __SUNPRO_CC && !defined __RESTRICT && !defined __restrict__
 # define _Restrict
 # define __restrict__
 #endif])