MacOS: Add workaround for std::variant

cmake/CompileFlags.cmake

@@ -193,4 +193,9 @@ macro(compile_flags)
         # rdynamic is used to get useful stack traces from crash reports.
         set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -rdynamic")
     endif()
+
+    if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+        # workaround for MacOS 10.13 and below which does not support std::variant, etc
+        add_definitions(-D_LIBCPP_DISABLE_AVAILABILITY)
+    endif()
 endmacro()

src/os/macosx/CMakeLists.txt

@@ -4,6 +4,7 @@ add_files(
     font_osx.h
     macos.h
     macos.mm
+    misc_osx.cpp
     osx_stdafx.h
     string_osx.cpp
     string_osx.h

src/os/macosx/misc_osx.cpp

@@ -0,0 +1,19 @@
+/*
+ * 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 misc_osx.cpp OS X misc functionality */
+
+#include "../../stdafx.h"
+
+#include "../../safeguards.h"
+
+#include <variant>
+
+/* See: https://stackoverflow.com/questions/52310835/xcode-10-call-to-unavailable-function-stdvisit/53868971#53868971 */
+const char* std::bad_variant_access::what() const noexcept {
+	return "bad_variant_access";
+}