Upgrade to libxml 2.9.2 and libxslt 1.1.28

third_party/libxml/src/include/libxml/xmlversion.h.in

Index: third_party/libxml/src/include/libxml/xmlversion.h.in
diff --git a/third_party/libxml/src/include/libxml/xmlversion.h.in b/third_party/libxml/src/include/libxml/xmlversion.h.in
index 95856541617009e037de8ea0cd8c3469992ff34b..b173be9e0730959491a494850e274617ed260090 100644
--- a/third_party/libxml/src/include/libxml/xmlversion.h.in
+++ b/third_party/libxml/src/include/libxml/xmlversion.h.in
@@ -98,6 +98,15 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
 #endif
 
 /**
+ * LIBXML_THREAD_ALLOC_ENABLED:
+ *
+ * Whether the allocation hooks are per-thread
+ */
+#if @WITH_THREAD_ALLOC@
+#define LIBXML_THREAD_ALLOC_ENABLED
+#endif
+
+/**
  * LIBXML_TREE_ENABLED:
  *
  * Whether the DOM like tree manipulation API support is configured in
@@ -391,6 +400,15 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
 #define LIBXML_ZLIB_ENABLED
 #endif
 
+/**
+ * LIBXML_LZMA_ENABLED:
+ *
+ * Whether the Lzma support is compiled in
+ */
+#if @WITH_LZMA@
+#define LIBXML_LZMA_ENABLED
+#endif
+
 #ifdef __GNUC__
 #ifdef HAVE_ANSIDECL_H
 #include <ansidecl.h>
@@ -403,7 +421,11 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
  */
 
 #ifndef ATTRIBUTE_UNUSED
-#define ATTRIBUTE_UNUSED __attribute__((unused))
+# if ((__GNUC__ > 2) || ((__GNUC__ == 2) && (__GNUC_MINOR__ >= 7)))
+#  define ATTRIBUTE_UNUSED __attribute__((unused))
+# else
+#  define ATTRIBUTE_UNUSED
+# endif
 #endif
 
 /**
@@ -413,7 +435,7 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
  */
 
 #ifndef LIBXML_ATTR_ALLOC_SIZE
-# if ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 3)))
+# if (!defined(__clang__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 3))))
 #  define LIBXML_ATTR_ALLOC_SIZE(x) __attribute__((alloc_size(x)))
 # else
 #  define LIBXML_ATTR_ALLOC_SIZE(x)