Check for custom free function in global destructor

Calling a custom deallocation function in the global destructor could
cause all kinds of unexpected problems. See for example

    https://github.com/sparklemotion/nokogiri/issues/2059

Only clean up if memory is managed with malloc/free.

parser.c

@@ -14696,6 +14696,11 @@ xmlCleanupParser(void) {
 static void
 ATTRIBUTE_DESTRUCTOR
 xmlDestructor(void) {
+    /*
+     * Calling custom deallocation functions in a destructor can cause
+     * problems, for example with Nokogiri.
+     */
+    if (xmlFree == free)
         xmlCleanupParser();
 }
 #endif