libxml2 2.9.10 fixes some things

[Joshua Root]

1. xml2-config now has a --dynamic option that can be used with --libs.
This should always be used when linking dynamically (as we almost always
are). With this flag available you don't have to switch to pkg-config to
avoid overlinking.

2. Along with the new upstream version, I added a patch to hide the icu
C++ API when not using C++11 or newer. That means dependents aren't
forced to use C++11 when they're just trying to use libxml2's C API and
don't even use icu.

So if you added a C++11 requirement to any ports purely because of build
failures in icu headers via libxml2, you can revert that now.

- Josh