use std::chrono::clock_cast when available

libi2pd/FS.cpp

@@ -260,12 +260,12 @@ namespace fs {
 		std::error_code ec;
 		auto t = std::filesystem::last_write_time (path, ec);
 		if (ec) return 0;
-/*#if __cplusplus >= 202002L // C++ 20 or higher
+#if __cpp_lib_chrono >= 201907L // std::chrono::clock_cast
 		const auto sctp = std::chrono::clock_cast<std::chrono::system_clock>(t);
-#else	*/	// TODO: wait until implemented
+#else
 		const auto sctp = std::chrono::time_point_cast<std::chrono::system_clock::duration>(
 		    t - decltype(t)::clock::now() + std::chrono::system_clock::now());
-/*#endif */
+#endif
 		return std::chrono::system_clock::to_time_t(sctp);
 #else
 		boost::system::error_code ec;