Add: Optional implementation of std::optional for pre-C++17 compilers.

Sourced from https://github.com/akrzemi1/Optional, Boost Software License, Version 1.0.

src/3rdparty/optional/ottd_optional.h

@@ -0,0 +1,33 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file ottd_optional.h Header to select between native. */
+
+#ifndef OTTD_OPTIONAL_H
+#define OTTD_OPTIONAL_H
+
+#if defined(__has_include)
+#	if __has_include(<version>)
+#		include <version>
+#	endif
+#endif
+
+#if (__cplusplus >= 201703L) || (defined(__cpp_lib_optional) && __cpp_lib_optional >= 201606L)
+
+/* Native std::optional. */
+#include <optional>
+namespace opt = std;
+
+#else
+
+/* No std::optional, use local copy instead. */
+#include "optional.hpp"
+namespace opt = std::experimental;
+
+#endif
+
+#endif /* OTTD_OPTIONAL_H */