Revert "Cleanup: use std::optional instead of custom implementation."

This reverts commit 484ea62a62.

src/3rdparty/optional/LICENSE_1_0.txt

@@ -0,0 +1,23 @@
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.

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 */

src/ini_type.h

@@ -12,7 +12,7 @@
 
 #include "fileio_type.h"
 #include <string>
-#include <optional>
+#include "3rdparty/optional/ottd_optional.h"
 
 #include <string>
 
@@ -27,7 +27,7 @@ enum IniGroupType {
 struct IniItem {
 	IniItem *next;                    ///< The next item in this group
 	std::string name;                 ///< The name of this item
-	std::optional<std::string> value; ///< The value of this item
+	opt::optional<std::string> value; ///< The value of this item
 	std::string comment;              ///< The comment associated with this item
 
 	IniItem(struct IniGroup *parent, const std::string &name);