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/zeroconf/ZeroconfAvahi.cxx

@@ -49,7 +49,7 @@ AvahiRegisterService(AvahiClient *c);
 static void
 AvahiGroupCallback(AvahiEntryGroup *g,
 		   AvahiEntryGroupState state,
-		   gcc_unused void *userdata)
+		   [[maybe_unused]] void *userdata)
 {
 	assert(g != nullptr);
 
@@ -149,7 +149,7 @@ AvahiRegisterService(AvahiClient *c)
 /* Callback when avahi changes state */
 static void
 MyAvahiClientCallback(AvahiClient *c, AvahiClientState state,
-		      gcc_unused void *userdata)
+		      [[maybe_unused]] void *userdata)
 {
 	assert(c != nullptr);
 

src/zeroconf/ZeroconfBonjour.cxx

@@ -48,7 +48,7 @@ public:
 
 protected:
 	/* virtual methods from class SocketMonitor */
-	bool OnSocketReady(gcc_unused unsigned flags) noexcept override {
+	bool OnSocketReady([[maybe_unused]] unsigned flags) noexcept override {
 		DNSServiceProcessResult(service_ref);
 		return true;
 	}
@@ -57,12 +57,12 @@ protected:
 static BonjourMonitor *bonjour_monitor;
 
 static void
-dnsRegisterCallback(gcc_unused DNSServiceRef sdRef,
-		    gcc_unused DNSServiceFlags flags,
+dnsRegisterCallback([[maybe_unused]] DNSServiceRef sdRef,
+		    [[maybe_unused]] DNSServiceFlags flags,
 		    DNSServiceErrorType errorCode, const char *name,
-		    gcc_unused const char *regtype,
-		    gcc_unused const char *domain,
-		    gcc_unused void *context)
+		    [[maybe_unused]] const char *regtype,
+		    [[maybe_unused]] const char *domain,
+		    [[maybe_unused]] void *context)
 {
 	if (errorCode != kDNSServiceErr_NoError) {
 		LogError(bonjour_domain,

src/zeroconf/ZeroconfGlue.cxx

@@ -50,7 +50,7 @@ static constexpr Domain zeroconf_domain("zeroconf");
 static int zeroconfEnabled;
 
 void
-ZeroconfInit(const ConfigData &config, gcc_unused EventLoop &loop)
+ZeroconfInit(const ConfigData &config, [[maybe_unused]] EventLoop &loop)
 {
 	const char *serviceName;