Update libxml to 2.7.7.

third_party/libxml/xmlstring.c

 /*
  * string.c : an XML string utilities module
  *
  * This module provides various utility functions for manipulating
  * the xmlChar* type. All functions named xmlStr* have been moved here
  * from the parser.c file (their original home). 
  *
  * See Copyright for the status of this software.
  *
  * UTF8 string routines from:
@@ skipping 348 matching lines @@
  * xmlStrcasestr:
  * @str:  the xmlChar * array (haystack)
  * @val:  the xmlChar to search (needle)
  *
  * a case-ignoring strstr for xmlChar's
  *
  * Returns the xmlChar * for the first occurrence or NULL.
  */
 
 const xmlChar *
-xmlStrcasestr(const xmlChar *str, xmlChar *val) {
+xmlStrcasestr(const xmlChar *str, const xmlChar *val) {
     int n;
     
     if (str == NULL) return(NULL);
     if (val == NULL) return(NULL);
     n = xmlStrlen(val);
 
     if (n == 0) return(str);
     while (*str != 0) { /* non input consuming */
         if (casemap[*str] == casemap[*val])
             if (!xmlStrncasecmp(str, val, n)) return(str);
@@ skipping 595 matching lines @@
                 if ( (*utf++ & 0xc0) != 0x80 )
                     return(NULL);
         }
     }
 
     return(xmlUTF8Strndup(utf, len));
 }
 
 #define bottom_xmlstring
 #include "elfgcchack.h"