gcc.h: re-add gcc_const and gcc_pure

Remove GLib dependency from some headers.

src/protocol/result.h

@@ -21,10 +21,9 @@
 #define MPD_PROTOCOL_RESULT_H
 
 #include "check.h"
+#include "gcc.h"
 #include "ack.h"
 
-#include <glib.h>
-
 struct client;
 
 extern const char *current_command;
@@ -37,7 +36,7 @@ void
 command_error_v(struct client *client, enum ack error,
 		const char *fmt, va_list args);
 
-G_GNUC_PRINTF(3, 4)
+gcc_fprintf_
 void
 command_error(struct client *client, enum ack error, const char *fmt, ...);