[patch] libstdc++/65473 Make <ciso646> define libstdc++ version macros.

[Jonathan Wakely <jwakely at redhat dot com>]

This change would allow including <ciso646> to be used to check for
__GLIBCXX__ and detect whether youre using libstdc++ or not. Howard
Hinnant recommends including that header for libc++ because it has no
other effects in C++.

We could make every <cxxx> header include <bits/c++config.h> so that
any of them can be used, but I can't be bothered doing that change!
This makes it work for the one header that is recommended to be used,
but of course that doesn't help people using older versions of
libstdc++, who still need to include some other header.

Is this worth doing?

commit 0ac33b5beb231efc94ce4f0288fad36047f0325e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 3 11:45:29 2015 +0100

    Make <ciso646> define libstdc++ version macros.
    
    	PR libstdc++/65473
    	* include/c/ciso646: Include <bits/c++config.h> and improve comment.
    	* include/c_global/ciso646: Likewise.
    	* include/c_std/ciso646: Likewise.

diff --git a/libstdc++-v3/include/c/ciso646 b/libstdc++-v3/include/c/ciso646
index 125f166..fb537f5 100644
--- a/libstdc++-v3/include/c/ciso646
+++ b/libstdc++-v3/include/c/ciso646
@@ -27,6 +27,6 @@
  *  in your programs, rather than any of the "*.h" implementation files.
  *
  *  This is the C++ version of the Standard C Library header @c iso646.h,
- *  and its contents are (mostly) the same as that header, but are all
- *  contained in the namespace @c std.
+ *  which is empty in C++.
  */
+#include <bits/c++config.h>
diff --git a/libstdc++-v3/include/c_global/ciso646 b/libstdc++-v3/include/c_global/ciso646
index 818db67..c59677a 100644
--- a/libstdc++-v3/include/c_global/ciso646
+++ b/libstdc++-v3/include/c_global/ciso646
@@ -27,7 +27,6 @@
  *  in your programs, rather than any of the @a *.h implementation files.
  *
  *  This is the C++ version of the Standard C Library header @c iso646.h,
- *  and its contents are (mostly) the same as that header, but are all
- *  contained in the namespace @c std (except for names which are defined
- *  as macros in C).
+ *  which is empty in C++.
  */
+#include <bits/c++config.h>
diff --git a/libstdc++-v3/include/c_std/ciso646 b/libstdc++-v3/include/c_std/ciso646
index 08cdf24..ab44488 100644
--- a/libstdc++-v3/include/c_std/ciso646
+++ b/libstdc++-v3/include/c_std/ciso646
@@ -27,7 +27,6 @@
  *  in your programs, rather than any of the @a *.h implementation files.
  *
  *  This is the C++ version of the Standard C Library header @c iso646.h,
- *  and its contents are (mostly) the same as that header, but are all
- *  contained in the namespace @c std (except for names which are defined
- *  as macros in C).
+ *  which is empty in C++.
  */
+#include <bits/c++config.h>