[libxml++] new and corrected patch to allow building without glibmm

[Vadim Zeitlin <vadz users sf net>]

 Hello again,

 as discussed previously, I've created a new patch which doesn't include
dummy glibmm/ustring.h file any more. It does include new
libxml++/xmlstring.h and libxml++/config.h files but hopefully it's ok
because I had already asked abotu the former and the latter can be useful
for any other compile time options too.

 I've also renamed LIBXML_USE_STD_STRING to LIBXMLPP_USE_STD_STRING and
replaced #ifdef's with a xmlpp::String::bytes() function which works with
either std::string or Glib::ustring.

 The patch itself is here: http://bugzilla.gnome.org/show_bug.cgi?id=320197
I've tested it and, to the best of my knowledge, it doesn't change anything,
API- or ABI-wise, if the new --without-ustring option is not used. When it
is used, however, libxml++ can now be built out of the box under Mac OS X
and Win32, unlike before.

 Please let me know if there is anything else I can do, thanks,
VZ