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/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