Log Message:
Update libxml++2 to 2.18.1.
Patch provided by Makoto Fujiwara by PR 36704.

Changes:
2.18.1:

2.18.0:

* Slight win32 build improvement, possibly.
  (Artur Wegele)
* Slight improvement to examples.
  (Murray Cumming)

2.17.2:

* Node: Adedd get_next_sibling() and get_previous_sibling().
  Bug #351867.
  (Nate Nielsen)

2.17.1:

* Node: Added get_parent() Bug #351876 (Patch by Nate Nielsen)
* TextReader: Added TextReader(const char* data) constructor, for
  parsing memory buffers. Bug #351215 (Patch by Nate Nielsen)
* Element::set_attribute(): Really return the new attribute.
  Bug #361950 (Patch by Nate Nielsen)
* Build: Added MS Visual Studio 2005 project files.
  (Cedric Gustin)