More libxml 2.12 fixes
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This merge request is automatically cherrypicked from !1657 (merged) from source branch https://source.coderefinery.org/ellert/arc/-/tree/libxml2-2.12 orginally made by @ellert .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/next
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More fixes to build with libxml2 2.12.