fix #3898 by adding -fPIC to xz-embedded

[ntorresalberto]

Fix #3898 by enabling -fPIC for the 3rd-party dep xz-embedded. @ricab

[ricab]

LGTM (modulo CLA).

Thanks again @ntorresalberto! Would you mind signing the CLA using the same Github handle / same email?

On top of that, part of our CI is currently blocked for external contributions. Would you mind if I pushed your branch into our repo and opened a new PR to replace this one? You would of course retain authorship of your commits and the new PR would be attributed to you in the description.

[ntorresalberto]

Sure, no problem.
I tried to sign the CLA, should I put your name as the "Canonical Project Manager or contact"?

[ricab]

Thank you @ntorresalberto, I guess you can, if it is mandatory. It is still not passing, but that may be just because it takes some time to sync. I'll check back this afternoon and we can follow up if it still fails.