use [[gnu::...]] attributes

src/player/CrossFade.cxx

@@ -22,7 +22,7 @@ CrossFadeSettings::CanCrossFadeSong(SignedSongTime total_time) const noexcept
 		duration < std::chrono::duration_cast<FloatDuration>(total_time);
 }
 
-gcc_pure
+[[gnu::pure]]
 static FloatDuration
 mixramp_interpolate(const char *ramp_list, float required_db) noexcept
 {

src/player/Thread.cxx

@@ -237,7 +237,7 @@ private:
 	 * Note: this function does not check if the decoder is already
 	 * finished.
 	 */
-	[[nodiscard]] gcc_pure
+	[[nodiscard]] [[gnu::pure]]
 	bool IsDecoderAtCurrentSong() const noexcept {
 		assert(pipe != nullptr);
 
@@ -249,7 +249,7 @@ private:
 	 * decoding it, or has finished doing it), and the player hasn't
 	 * switched to that song yet.
 	 */
-	[[nodiscard]] gcc_pure
+	[[nodiscard]] [[gnu::pure]]
 	bool IsDecoderAtNextSong() const noexcept {
 		return dc.pipe != nullptr && !IsDecoderAtCurrentSong();
 	}