Disable std::string and std::wstring extern templates for C++17

	* include/bits/basic_string.tcc: Disable explicit instantiation
	declarations for C++17.

From-SVN: r238959

libstdc++-v3/ChangeLog

 2016-08-01  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/basic_string.tcc: Disable explicit instantiation
+	declarations for C++17.
+
 	* testsuite/23_containers/unordered_map/requirements/53339.cc: Remove
 	ignored "xfail" from { dg-do compile { xfail selector } } directive.
 	* testsuite/23_containers/unordered_multimap/requirements/53339.cc:

libstdc++-v3/include/bits/basic_string.tcc

@@ -1569,7 +1569,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // Inhibit implicit instantiations for required instantiations,
   // which are defined via explicit instantiations elsewhere.
-#if _GLIBCXX_EXTERN_TEMPLATE > 0
+#if _GLIBCXX_EXTERN_TEMPLATE > 0 && __cplusplus <= 201402L
   extern template class basic_string<char>;
   extern template
     basic_istream<char>&