Disable media_downloader example on old macOS

examples/CMakeLists.txt

@@ -10,8 +10,13 @@ include_directories(../tests)
 add_executable(room_feed room_feed.cpp)
 target_link_libraries(room_feed MatrixClient::MatrixClient)
 
-add_executable(media_downloader media_downloader.cpp)
-target_link_libraries(media_downloader MatrixClient::MatrixClient)
+include(CheckIncludeFileCXX)
+CHECK_INCLUDE_FILE_CXX("filesystem" HAVE_STD_FILESYSTEM)
+
+if (HAVE_STD_FILESYSTEM)
+  add_executable(media_downloader media_downloader.cpp)
+  target_link_libraries(media_downloader MatrixClient::MatrixClient)
+endif()
 
 add_executable(simple_bot simple_bot.cpp)
 target_link_libraries(simple_bot MatrixClient::MatrixClient)