libstdc++: Add #error to some files that depend on a specific standard mode

Give more explicit errors if these files are not built with the correct
-std options.

libstdc++-v3/ChangeLog:

	* src/c++98/locale_init.cc: Require C++11.
	* src/c++98/localename.cc: Likewise.
	* src/c++98/misc-inst.cc: Require C++98.

libstdc++-v3/src/c++98/locale_init.cc

@@ -20,6 +20,10 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#if __cplusplus != 201103L
+# error This file must be compiled as C++11
+#endif
+
 #define _GLIBCXX_USE_CXX11_ABI 1
 #include <clocale>
 #include <cstring>

libstdc++-v3/src/c++98/localename.cc

@@ -20,6 +20,10 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#if __cplusplus != 201103L
+# error This file must be compiled as C++11
+#endif
+
 #define _GLIBCXX_USE_CXX11_ABI 1
 #include <clocale>
 #include <cstring>

libstdc++-v3/src/c++98/misc-inst.cc

@@ -26,6 +26,10 @@
 // ISO C++ 14882:
 //
 
+#if __cplusplus != 199711L
+# error This file must be compiled as C++98
+#endif
+
 #define _GLIBCXX_USE_CXX11_ABI 1
 #define _GLIBCXX_DISAMBIGUATE_REPLACE_INST 1
 #include <string>