Provide a way to disable use of extern templates in libc++.  This is intended for the clients of libc++, not the libc++ build.  The dylib should always contain the extern templates.  To disable the client needs to put -D'_LIBCPP_EXTERN_TEMPLATE(...)=' on the command line.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@167486 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index 4e69964..2bbe1d9 100644
--- a/include/__config
+++ b/include/__config
@@ -427,6 +427,10 @@
 #define _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(x)
 #endif  // _LIBCPP_HAS_NO_STRONG_ENUMS
 
+#ifndef _LIBCPP_EXTERN_TEMPLATE
+#define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
+#endif
+
 #if __APPLE__ || __FreeBSD__ || _WIN32 || __sun__
 #define _LIBCPP_LOCALE__L_EXTENSIONS 1
 #endif