Fix std::optional related compilation errors on some platforms

src/strings.cpp

@@ -2549,7 +2549,7 @@ class LanguagePackGlyphSearcher : public MissingGlyphSearcher {
 		return FS_NORMAL;
 	}
 
-	std::optional<std::string_view> NextString() override
+	opt::optional<std::string_view> NextString() override
 	{
 		if (this->i >= TEXT_TAB_END) return std::nullopt;
 

src/strings_func.h

@@ -15,6 +15,7 @@
 #include "gfx_type.h"
 #include "core/bitmath_func.hpp"
 #include "vehicle_type.h"
+#include "3rdparty/optional/ottd_optional.h"
 
 /**
  * Extract the StringTab from a StringID.
@@ -266,7 +267,7 @@ public:
 	 * Get the next string to search through.
 	 * @return The next string or nullopt if there is none.
 	 */
-	virtual std::optional<std::string_view> NextString() = 0;
+	virtual opt::optional<std::string_view> NextString() = 0;
 
 	/**
 	 * Get the default (font) size of the string.

src/textfile_gui.cpp

@@ -194,7 +194,7 @@ void TextfileWindow::SetupScrollbars(bool force_reflow)
 	return FS_MONO;
 }
 
-/* virtual */ std::optional<std::string_view> TextfileWindow::NextString()
+/* virtual */ opt::optional<std::string_view> TextfileWindow::NextString()
 {
 	if (this->search_iterator >= this->lines.size()) return std::nullopt;
 

src/textfile_gui.h

@@ -14,6 +14,7 @@
 #include "strings_func.h"
 #include "textfile_type.h"
 #include "window_gui.h"
+#include "3rdparty/optional/ottd_optional.h"
 
 const char *GetTextfile(TextfileType type, Subdirectory dir, const char *filename);
 
@@ -36,7 +37,7 @@ struct TextfileWindow : public Window, MissingGlyphSearcher {
 
 	void Reset() override;
 	FontSize DefaultSize() override;
-	std::optional<std::string_view> NextString() override;
+	opt::optional<std::string_view> NextString() override;
 	bool Monospace() override;
 	void SetFontNames(FontCacheSettings *settings, const char *font_name, const void *os_data) override;