Fix compile error with -Werror=incompatible-function-pointer-types and old libxml2

libxml2 prior to 2.9.8 had a different signature for xmlHashScanner.
This signature changed in e03f0a199a
Use an #if to work around the incompatible signature.

Closes GH-12326.

NEWS

@@ -13,6 +13,10 @@ PHP                                                                        NEWS
 - DOM:
   . Restore old namespace reconciliation behaviour. (nielsdos)
 
+- LibXML:
+  . Fix compile error with -Werror=incompatible-function-pointer-types and
+    old libxml2. (nielsdos)
+
 - MySQLnd:
   . Fixed bug GH-12297 (PHP Startup: Invalid library (maybe not a PHP library)
     'mysqlnd.so' in Unknown on line). (nielsdos)

ext/dom/dom_iterators.c

@@ -38,6 +38,7 @@ struct _notationIterator {
 	xmlNotation *notation;
 };
 
+/* Function pointer typedef changed in 2.9.8, see https://github.com/GNOME/libxml2/commit/e03f0a199a67017b2f8052354cf732b2b4cae787 */
 #if LIBXML_VERSION >= 20908
 static void itemHashScanner (void *payload, void *data, const xmlChar *name) /* {{{ */
 #else

ext/libxml/libxml.c

@@ -136,7 +136,12 @@ PHP_LIBXML_API void php_libxml_set_old_ns(xmlDocPtr doc, xmlNsPtr ns)
 	php_libxml_set_old_ns_list(doc, ns, ns);
 }
 
+/* Function pointer typedef changed in 2.9.8, see https://github.com/GNOME/libxml2/commit/e03f0a199a67017b2f8052354cf732b2b4cae787 */
+#if LIBXML_VERSION >= 20908
 static void php_libxml_unlink_entity(void *data, void *table, const xmlChar *name)
+#else
+static void php_libxml_unlink_entity(void *data, void *table, xmlChar *name)
+#endif
 {
 	xmlEntityPtr entity = data;
 	if (entity->_private != NULL) {