build: allow clients to provide their own pugixml

[rpatters1]

This PR gates the inclusion of the local vendored pugixml on whether another already exists. It allows a client project that also uses pugixml to include mx without creating a conflict.

Resolves #242

[webern]

Thanks for working on this. As written it won’t quite work because I have dumb header paths for the pugi headers. There’s a small fix needed in the Cmake file and the include paths need to change at all the sites where pugi is referenced.

I’m on a mobile phone right now. Later I can add a commit to this PR or you can do it if you get there first:

git remote add upstream https://github.com/webern/mx.git
git fetch upstream pugi-headers
git cherry-pick 99dc0a9

Also the CI failures are something I need to fix. It looks like my comment posting workflows for code coverage don’t work for anyone but me.

[rpatters1]

It was working for me, weirdly, so I never looked at the include sites. Thank you for the quick response. I updated the PR with your change.

[webern]

It was working for me, weirdly, so I never looked at the include sites. Thank you for the quick response. I updated the PR with your change.

I think it worked because your version of pugixml and the vendored version were the same, or at least close enough that the headers did not differ in any binary-affecting way. So I think when you were building mx you were reading the vendored headers but linking the cpp's you had built outside.

Anyway, I had forgotten to update one thing so did a force push to that third commit (mine). And checking CI 👍

[webern]

Nice, thank you for fixing this!