Fixing compilation error when building with emscripten.

This change was previously done in //third_party in CL 108656107 but never made it to the open source project and was overwritten in an update.

src/google/protobuf/map.h

@@ -545,7 +545,9 @@ class Map {
     }
 
 #if __cplusplus >= 201103L && !defined(GOOGLE_PROTOBUF_OS_APPLE) && \
-    !defined(GOOGLE_PROTOBUF_OS_NACL) && !defined(GOOGLE_PROTOBUF_OS_ANDROID)
+    !defined(GOOGLE_PROTOBUF_OS_NACL) &&                            \
+    !defined(GOOGLE_PROTOBUF_OS_ANDROID) &&                         \
+    !defined(GOOGLE_PROTOBUF_OS_EMSCRIPTEN)
     template<class NodeType, class... Args>
     void construct(NodeType* p, Args&&... args) {
       // Clang 3.6 doesn't compile static casting to void* directly. (Issue #1266)