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>

src/event/Loop.cxx

@@ -271,7 +271,7 @@ EventLoop::HandleDeferred() noexcept
 }
 
 bool
-EventLoop::OnSocketReady(gcc_unused unsigned flags) noexcept
+EventLoop::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 {
 	assert(IsInside());
 

src/event/PollGroupEpoll.hxx

@@ -89,7 +89,7 @@ public:
 		return epoll.Remove(fd);
 	}
 
-	bool Abandon(gcc_unused int fd) noexcept {
+	bool Abandon([[maybe_unused]] int fd) noexcept {
 		// Nothing to do in this implementation.
 		// Closed descriptors are automatically unregistered.
 		return true;

src/event/ServerSocket.cxx

@@ -161,7 +161,7 @@ ServerSocket::OneServerSocket::Accept() noexcept
 }
 
 bool
-ServerSocket::OneServerSocket::OnSocketReady(gcc_unused unsigned flags) noexcept
+ServerSocket::OneServerSocket::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 {
 	Accept();
 	return true;