Prevent Pugi Symbol Conflicts

[webern]

Per @p-groarke in #53

Embedding pugixml in your repo like you did will make mx unusable to anyone who links directly with pugixml in their project (duplicate symbols). You can google "dependency hell" for more info (yes, that's the real name). To fix that, you could use something super robust like conan to install pugi when you build mx. Or you can use the less robust ExternalProject_Add as well. I would be happy to send a PR with any of those solutions.

True, embedding could be retained but this could be fixed by patching the pugi namespace to something else.

See webern/ezxml#6