Xerces-C is the C++ XML Parser from the Apache Software Foundation. It is used for all XML processing in OpenSAML and Shibboleth.
Project Website: http://xml.apache.org/xerces-c
You can use the standard version, but the Internet2-patched version (2.7.1) contains some improvements that will eventually show up in Xerces 3.0, which is not out yet.
For now, build notes can be found in the BuildingShibboleth topic.