include: Don't redefine ATTRIBUTE_UNUSED

Author: nwellnhof

include/libxml/xmlexports.h

@@ -41,10 +41,12 @@
  * Attributes
  */
 
-#if __GNUC__ * 100 + __GNUC_MINOR__ >= 207
-  #define ATTRIBUTE_UNUSED __attribute__((unused))
-#else
-  #define ATTRIBUTE_UNUSED
+#ifndef ATTRIBUTE_UNUSED
+  #if __GNUC__ * 100 + __GNUC_MINOR__ >= 207
+    #define ATTRIBUTE_UNUSED __attribute__((unused))
+  #else
+    #define ATTRIBUTE_UNUSED
+  #endif
 #endif
 
 #if !defined(__clang__) && (__GNUC__ * 100 + __GNUC_MINOR__ >= 403)