which pulls in the headers for the various parsers if the appropriate macros are
defined - USE_EXPAT, USE_XERCES, USE_LIBXML2, USE_MSXML.
You can define any or all of them, or none should you so wish.
It also sets SAX::XMLReader as a default class - ie if you define one of the above macros,
SAX::XMLReader<stringT> will become a synonym for SAX::expat_wrapper<stringT> or whatever.
