Fix incompatibilities with libxml2 >= 2.12.0.

libxml2 changed the required signature of error handler callbacks
to make the passed xmlError struct "const".  This is causing build
failures on buildfarm member caiman, and no doubt will start showing
up in the field quite soon.  Add a version check to adjust the
declaration of xml_errorHandler() according to LIBXML_VERSION.

2.12.x also produces deprecation warnings for contrib/xml2/xpath.c's
assignment to xmlLoadExtDtdDefaultValue.  I see no good reason for
that to still be there, seeing that we disabled external DTDs (at a
lower level) years ago for security reasons.  Let's just remove it.

Back-patch to all supported branches, since they might all get built
with newer libxml2 once it gets a bit more popular.  (The back
branches produce another deprecation warning about xpath.c's use of
xmlSubstituteEntitiesDefault().  We ought to consider whether to
back-patch all or part of commit 65c5864 to silence that.  It's
less urgent though, since it won't break the buildfarm.)

Discussion: https://postgr.es/m/1389505.1706382262@sss.pgh.pa.us

Author: tglsfdc

contrib/xml2/xpath.c

@@ -75,7 +75,6 @@ pgxml_parser_init(PgXmlStrictness strictness)
 	xmlInitParser();
 
 	xmlSubstituteEntitiesDefault(1);
-	xmlLoadExtDtdDefaultValue = 1;
 
 	return xmlerrcxt;
 }

src/backend/utils/adt/xml.c

@@ -66,6 +66,16 @@
 #if LIBXML_VERSION >= 20704
 #define HAVE_XMLSTRUCTUREDERRORCONTEXT 1
 #endif
+
+/*
+ * libxml2 2.12 decided to insert "const" into the error handler API.
+ */
+#if LIBXML_VERSION >= 21200
+#define PgXmlErrorPtr const xmlError *
+#else
+#define PgXmlErrorPtr xmlErrorPtr
+#endif
+
 #endif							/* USE_LIBXML */
 
 #include "access/htup_details.h"
@@ -123,7 +133,7 @@ static xmlParserInputPtr xmlPgEntityLoader(const char *URL, const char *ID,
 										   xmlParserCtxtPtr ctxt);
 static void xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
 						int sqlcode, const char *msg);
-static void xml_errorHandler(void *data, xmlErrorPtr error);
+static void xml_errorHandler(void *data, PgXmlErrorPtr error);
 static int	errdetail_for_xml_code(int code);
 static void chopStringInfoNewlines(StringInfo str);
 static void appendStringInfoLineSeparator(StringInfo str);
@@ -2002,7 +2012,7 @@ xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
  * Error handler for libxml errors and warnings
  */
 static void
-xml_errorHandler(void *data, xmlErrorPtr error)
+xml_errorHandler(void *data, PgXmlErrorPtr error)
 {
 	PgXmlErrorContext *xmlerrcxt = (PgXmlErrorContext *) data;
 	xmlParserCtxtPtr ctxt = (xmlParserCtxtPtr) error->ctxt;