Roll libxml to 2.9.3

third_party/libxml/mac/include/libxml/xmlversion.h

Index: third_party/libxml/mac/include/libxml/xmlversion.h
diff --git a/third_party/libxml/mac/include/libxml/xmlversion.h b/third_party/libxml/mac/include/libxml/xmlversion.h
index 6fd386de05bb659f249e4dd329827a629d0e4a78..bc7b4c48782c8e6f1a1aafce86a8641839450eb9 100644
--- a/third_party/libxml/mac/include/libxml/xmlversion.h
+++ b/third_party/libxml/mac/include/libxml/xmlversion.h
@@ -29,21 +29,21 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
  *
  * the version string like "1.2.3"
  */
-#define LIBXML_DOTTED_VERSION "2.9.2"
+#define LIBXML_DOTTED_VERSION "2.9.3"
 
 /**
  * LIBXML_VERSION:
  *
  * the version number: 1.2.3 value is 10203
  */
-#define LIBXML_VERSION 20902
+#define LIBXML_VERSION 20903
 
 /**
  * LIBXML_VERSION_STRING:
  *
  * the version number string, 1.2.3 value is "10203"
  */
-#define LIBXML_VERSION_STRING "20902"
+#define LIBXML_VERSION_STRING "20903"
 
 /**
  * LIBXML_VERSION_EXTRA:
@@ -58,7 +58,7 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
  * Macro to check that the libxml version in use is compatible with
  * the version the software has been compiled against
  */
-#define LIBXML_TEST_VERSION xmlCheckVersion(20902);
+#define LIBXML_TEST_VERSION xmlCheckVersion(20903);
 
 #ifndef VMS
 #if 0
@@ -435,7 +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)