Looking for libxml++-5.0 before falling back to libxml++-2.6.

[gian21391]

As pointed out in #193, the usage of libxml++-5.0 gives no problems.
I have successfully tested the usage of a statically linked version libxml++-5.0, so why not use it if it is available in the system?

[phako]

I am doing that for Yocto currently - which distribution in the wild did you find that actually used libxml++5?

[gian21391]

I'm using libxml++5 shipped with vcpkg.
(PS: I've also packaged this repository as a library in my registry to be used in my project).

[phako]

The slight problem we have is that we would need to support it in all other depending projects as well and that isn't plain sailing for the rest, e.g. https://github.com/ChimeraTK/meta-chimeratk/blob/master/recipes-applicationcore/applicationcore/applicationcore/0001-Bump-libxml-to-5.0.patch and https://github.com/ChimeraTK/meta-chimeratk/blob/master/recipes-applicationcore/controlsystemadapter/controlsystemadapter/0001-Bump-libxml-version-to-5.0.patch