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/db/Interface.hxx

@@ -126,8 +126,8 @@ public:
 	 *
 	 * @return the job id or 0 if not implemented
 	 */
-	virtual unsigned Update(gcc_unused const char *uri_utf8,
-				gcc_unused bool discard) {
+	virtual unsigned Update([[maybe_unused]] const char *uri_utf8,
+				[[maybe_unused]] bool discard) {
 		/* not implemented: return 0 */
 		return 0;
 	}

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -575,7 +575,7 @@ ProxyDatabase::Disconnect() noexcept
 }
 
 bool
-ProxyDatabase::OnSocketReady(gcc_unused unsigned flags) noexcept
+ProxyDatabase::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 {
 	assert(connection != nullptr);
 

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -70,7 +70,7 @@ inline SimpleDatabase::SimpleDatabase(const ConfigBlock &block)
 
 inline SimpleDatabase::SimpleDatabase(AllocatedPath &&_path,
 #ifndef ENABLE_ZLIB
-				      gcc_unused
+				      [[maybe_unused]]
 #endif
 				      bool _compress) noexcept
 	:Database(simple_db_plugin),
@@ -85,7 +85,7 @@ inline SimpleDatabase::SimpleDatabase(AllocatedPath &&_path,
 
 DatabasePtr
 SimpleDatabase::Create(EventLoop &, EventLoop &,
-		       gcc_unused DatabaseListener &listener,
+		       [[maybe_unused]] DatabaseListener &listener,
 		       const ConfigBlock &block)
 {
 	return std::make_unique<SimpleDatabase>(block);
@@ -248,7 +248,7 @@ SimpleDatabase::GetSong(const char *uri) const
 }
 
 void
-SimpleDatabase::ReturnSong(gcc_unused const LightSong *song) const noexcept
+SimpleDatabase::ReturnSong([[maybe_unused]] const LightSong *song) const noexcept
 {
 	assert(song != nullptr);
 	assert(song == prefixed_light_song || song == &light_song.Get());

src/db/plugins/upnp/UpnpDatabasePlugin.cxx

@@ -145,7 +145,7 @@ private:
 
 DatabasePtr
 UpnpDatabase::Create(EventLoop &, EventLoop &io_event_loop,
-		     gcc_unused DatabaseListener &listener,
+		     [[maybe_unused]] DatabaseListener &listener,
 		     const ConfigBlock &) noexcept
 {
 	return std::make_unique<UpnpDatabase>(io_event_loop);

src/db/update/InotifySource.cxx

@@ -31,7 +31,7 @@
 #include <sys/inotify.h>
 
 bool
-InotifySource::OnSocketReady(gcc_unused unsigned flags) noexcept
+InotifySource::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 {
 	uint8_t buffer[4096];
 	static_assert(sizeof(buffer) >= sizeof(struct inotify_event) + NAME_MAX + 1,

src/db/update/InotifyUpdate.cxx

@@ -236,7 +236,7 @@ WatchDirectory::GetDepth() const noexcept
 
 static void
 mpd_inotify_callback(int wd, unsigned mask,
-		     gcc_unused const char *name, gcc_unused void *ctx)
+		     [[maybe_unused]] const char *name, [[maybe_unused]] void *ctx)
 {
 	WatchDirectory *directory;
 

src/db/update/Walk.hxx

@@ -111,10 +111,10 @@ private:
 
 
 #else
-	bool UpdateArchiveFile(gcc_unused Directory &directory,
-			       gcc_unused const char *name,
-			       gcc_unused const char *suffix,
-			       gcc_unused const StorageFileInfo &info) noexcept {
+	bool UpdateArchiveFile([[maybe_unused]] Directory &directory,
+			       [[maybe_unused]] const char *name,
+			       [[maybe_unused]] const char *suffix,
+			       [[maybe_unused]] const StorageFileInfo &info) noexcept {
 		return false;
 	}
 #endif