[committed][_GLIBCXX_INLINE_VERSION] Fix constract violation

[François Dumont]

This fixes handle_contract_violation under versioned namespace mode.
Tested under Linux x64 and confirmed to also fix Darwin build.

libstdc++: [_GLIBCXX_INLINE_VERSION] Provide handle_contract_violation 
symbol

libstdc++-v3/ChangeLog:

        * src/experimental/contract.cc
        [_GLIBCXX_INLINE_VERSION](handle_contract_violation): Provide 
symbol
        without version namespace decoration for gcc.

François
diff --git a/libstdc++-v3/src/experimental/contract.cc b/libstdc++-v3/src/experimental/contract.cc
index 504a6c041f1..d550b49c4eb 100644
--- a/libstdc++-v3/src/experimental/contract.cc
+++ b/libstdc++-v3/src/experimental/contract.cc
@@ -67,3 +67,11 @@ handle_contract_violation (const std::experimental::contract_violation &violatio
   std::cerr << std::endl;
 #endif
 }
+
+#if _GLIBCXX_INLINE_VERSION
+// Provide symbol without version namespace decoration for gcc.
+extern "C" __attribute__ ((weak)) void
+_Z25handle_contract_violationRKNSt12experimental18contract_violationE
+(const std::experimental::contract_violation &violation)
+{ handle_contract_violation(violation); }
+#endif