Comment 1 for bug 320937

> Including this in a c++ source file as:
> #include <libxml++-2.6/libxml++/libxml++.h>

But you shouldn't do that. You should use pkg-config to give you the include path, and just use
#include <libxml++/libxml++.h>

This is entirely normal for libraries which plan to have parallel-installed versions, as all libraries should.

This may help:
http://libxmlplusplus.sourceforge.net/docs/manual/html/index.html#id2504511

(I am the upstream maintainer.)