RFA: v3 PATCH to add on_quick_exit/quick_exit to std

[Paolo Carlini]

... or the below, just in case an interesting system provides the 
*quick_exit functions but doesn't define _GLIBCXX_USE_C99.

Paolo.

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Index: testsuite/18_support/quick_exit/quick_exit.cc
===================================================================
--- testsuite/18_support/quick_exit/quick_exit.cc	(revision 196056)
+++ testsuite/18_support/quick_exit/quick_exit.cc	(working copy)
@@ -25,7 +25,9 @@
 
 void handler()
 {
+#if _GLIBCXX_USE_C99
   std::_Exit(0);
+#endif
 }
 
 void wrong_handler()
@@ -35,9 +37,11 @@
 
 int main()
 {
+#if _GLIBCXX_USE_C99
 #if defined(_GLIBCXX_HAVE_AT_QUICK_EXIT) && defined(_GLIBCXX_HAVE_QUICK_EXIT)
   std::at_quick_exit (handler);
   std::atexit (wrong_handler);
   std::quick_exit (1);
 #endif
+#endif
 }