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/valarray b/include/valarray
index 4091d0f..c56dd12 100644
--- a/include/valarray
+++ b/include/valarray
@@ -4770,9 +4770,9 @@
     return __v.__end_;
 }
 
-extern template valarray<size_t>::valarray(size_t);
-extern template valarray<size_t>::~valarray();
-extern template void valarray<size_t>::resize(size_t, size_t);
+_LIBCPP_EXTERN_TEMPLATE(valarray<size_t>::valarray(size_t))
+_LIBCPP_EXTERN_TEMPLATE(valarray<size_t>::~valarray())
+_LIBCPP_EXTERN_TEMPLATE(void valarray<size_t>::resize(size_t, size_t))
 
 _LIBCPP_END_NAMESPACE_STD