Fix YAML include dirs (should not be relative path resolving to /usr/include)

[davidrohr]

Fix usage of YAML-CPP include dirs, when they are in /usr/include.
Otherwise, CMake will add -isystem /usr/[SOMETHING]/../include to the gcc command line, which breaks GCC for some known weird reason.

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Checklist:

• Rebased against dev branch
• My name is in the resp. CONTRIBUTORS/AUTHORS file
• Followed the seven rules of great commit messages

[dennisklein]

@davidrohr It wouldn't let me force push to your branch. If you give me a thumbs up in #911, I will merge that, or if you want to get my changes from my fork (branch davidrohr_PR2) in this PR, I can merge this one. However you prefer :) Thx for the contrib!

[davidrohr]

Weird, it is definitely marked as "allow edits from maintainers", no idea what is the problem.

[dennisklein]

"allow edits from maintainers", no idea what is the problem.

Had this problem many times in the past with Alexey too. It seems to be depending on some additional condition, maybe we need to be member of the same org or just force-pushes are not possible or the checkbox needs to be checked right away? sometimes it works for me, sometimes not..