Update libxml to 2.7.7.

third_party/libxml/linux/xml2-config

 #! /bin/sh
 
 prefix=/usr/local
 exec_prefix=${prefix}
 includedir=${prefix}/include
 libdir=${exec_prefix}/lib
 
 usage()
 {
     cat <<EOF
@@ skipping 40 matching lines @@
     --exec-prefix=*)
       exec_prefix=$optarg
       libdir=$exec_prefix/lib
       ;;
 
     --exec-prefix)
       echo $exec_prefix
       ;;
 
     --version)
-        echo 2.6.32
+        echo 2.7.7
         exit 0
         ;;
 
     --help)
         usage 0
         ;;
 
     --cflags)
         echo -I${includedir}/libxml2 
         ;;
@@ skipping 25 matching lines @@
 
     *)
         usage
         exit 1
         ;;
     esac
     shift
 done
 
 exit 0