make 'no syntax' warning useful by showing which file

src/google/protobuf/compiler/parser.cc

@@ -544,9 +544,9 @@ bool Parser::Parse(io::Tokenizer* input, FileDescriptorProto* file) {
       // Store the syntax into the file.
       if (file != NULL) file->set_syntax(syntax_identifier_);
     } else if (!stop_after_syntax_identifier_) {
-      GOOGLE_LOG(WARNING) << "No syntax specified for the proto file. "
-                   << "Please use 'syntax = \"proto2\";' or "
-                   << "'syntax = \"proto3\";' to specify a syntax "
+      GOOGLE_LOG(WARNING) << "No syntax specified for the proto file: "
+                   << file->name() << ". Please use 'syntax = \"proto2\";' "
+                   << "or 'syntax = \"proto3\";' to specify a syntax "
                    << "version. (Defaulted to proto2 syntax.)";
       syntax_identifier_ = "proto2";
     }