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/encoder/EncoderInterface.hxx

@@ -84,7 +84,7 @@ public:
 	 *
 	 * @param tag the tag object
 	 */
-	virtual void SendTag(gcc_unused const Tag &tag) {
+	virtual void SendTag([[maybe_unused]] const Tag &tag) {
 	}
 
 	/**

src/encoder/plugins/FlacEncoderPlugin.cxx

@@ -69,8 +69,8 @@ private:
 	static FLAC__StreamEncoderWriteStatus WriteCallback(const FLAC__StreamEncoder *,
 							    const FLAC__byte data[],
 							    size_t bytes,
-							    gcc_unused unsigned samples,
-							    gcc_unused unsigned current_frame,
+							    [[maybe_unused]] unsigned samples,
+							    [[maybe_unused]] unsigned current_frame,
 							    void *client_data) noexcept {
 		auto &encoder = *(FlacEncoder *)client_data;
 		encoder.output_buffer.Append((const uint8_t *)data, bytes);

src/encoder/plugins/NullEncoderPlugin.cxx

@@ -49,7 +49,7 @@ public:
 };
 
 static PreparedEncoder *
-null_encoder_init(gcc_unused const ConfigBlock &block)
+null_encoder_init([[maybe_unused]] const ConfigBlock &block)
 {
 	return new PreparedNullEncoder();
 }

src/encoder/plugins/WaveEncoderPlugin.cxx

@@ -98,7 +98,7 @@ fill_wave_header(WaveHeader *header, int channels, int bits,
 }
 
 static PreparedEncoder *
-wave_encoder_init(gcc_unused const ConfigBlock &block)
+wave_encoder_init([[maybe_unused]] const ConfigBlock &block)
 {
 	return new PreparedWaveEncoder();
 }