Add ASF license header

src/main/include/log4cxx/helpers/optional.h

@@ -1,10 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef LOG4CXX_OPTIONAL_HDR_
+#define LOG4CXX_OPTIONAL_HDR_
+
 #ifdef __has_include                           // Check if __has_include is present
 #  if __has_include(<optional>)                // Check for a standard version
 #    include <optional>
 #    if defined(__cpp_lib_optional)            // C++ >= 17
 namespace LOG4CXX_NS { template< class T > using Optional = std::optional<T>; }
 #define LOG4CXX_HAS_STD_OPTIONAL 1
-#endif
+#    endif
 #  elif __has_include(<experimental/optional>) // Check for an experimental version
 #    include <experimental/optional>
 namespace LOG4CXX_NS { template< class T > using Optional = std::experimental::optional<T>; }
@@ -38,3 +58,5 @@ class Optional : private std::pair<bool, T>
 };
 } // namespace LOG4CXX_NS
 #endif
+
+#endif // LOG4CXX_OPTIONAL_HDR_