Convert libglnx to subtree

[bbhtt]

No description provided.

[bbhtt]

This is at same commit added with:

git subtree add --prefix=subprojects/libglnx libglnx 202b294e6079e23242e65e0426f8639841d1210b --squash

Updating looks like this:

git subtree pull --prefix=subprojects/libglnx libglnx master --squash

[bbhtt]

Merged manually through git, GitHub doesn't allow to merge this.

git checkout main
rm -rf subprojects/libglnx
git merge --no-ff bbhtt/libglnx-subtree-2
git push