Avoid two consecutive underscores in macro name.

Fixes https://github.com/google/protobuf/issues/3268

src/google/protobuf/compiler/cpp/cpp_file.cc

@@ -1145,8 +1145,8 @@ void FileGenerator::GenerateTopHeaderGuard(io::Printer* printer,
       "// Generated by the protocol buffer compiler.  DO NOT EDIT!\n"
       "// source: $filename$\n"
       "\n"
-      "#ifndef PROTOBUF_$filename_identifier$__INCLUDED\n"
-      "#define PROTOBUF_$filename_identifier$__INCLUDED\n"
+      "#ifndef PROTOBUF_$filename_identifier$_INCLUDED\n"
+      "#define PROTOBUF_$filename_identifier$_INCLUDED\n"
       "\n"
       "#include <string>\n",
       "filename", file_->name(), "filename_identifier", filename_identifier);
@@ -1156,7 +1156,7 @@ void FileGenerator::GenerateTopHeaderGuard(io::Printer* printer,
 void FileGenerator::GenerateBottomHeaderGuard(
     io::Printer* printer, const string& filename_identifier) {
   printer->Print(
-    "#endif  // PROTOBUF_$filename_identifier$__INCLUDED\n",
+    "#endif  // PROTOBUF_$filename_identifier$_INCLUDED\n",
     "filename_identifier", filename_identifier);
 }