remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

test/run_avahi.cxx

@@ -26,7 +26,7 @@
 unsigned listen_port = 1234;
 
 int
-main(gcc_unused int argc, gcc_unused char **argv)
+main([[maybe_unused]] int argc, [[maybe_unused]] char **argv)
 {
 	EventLoop event_loop;
 	const ShutdownHandler shutdown_handler(event_loop);