C++ PATCH to update __cplusplus for C++17

[Jason Merrill]

This is the value discussed at the Kona meeting.

commit 6a8c9d106fd69e2aa0dd54fe856b7083d7988c7e
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu Mar 16 13:06:48 2017 -0400

            * init.c (cpp_init_builtins): Update __cplusplus for C++17.

diff --git a/libcpp/init.c b/libcpp/init.c
index 0679b7f..1477673 100644
--- a/libcpp/init.c
+++ b/libcpp/init.c
@@ -499,7 +499,7 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
     {
       if (CPP_OPTION (pfile, lang) == CLK_CXX1Z
          || CPP_OPTION (pfile, lang) == CLK_GNUCXX1Z)
-       _cpp_define_builtin (pfile, "__cplusplus 201500L");
+       _cpp_define_builtin (pfile, "__cplusplus 201703L");
       else if (CPP_OPTION (pfile, lang) == CLK_CXX14
          || CPP_OPTION (pfile, lang) == CLK_GNUCXX14)
        _cpp_define_builtin (pfile, "__cplusplus 201402L");