-- extensions: don't rely on __GNUC__ in clang builds

[MariusBgm]

Subject

The clang builds relied on GNUC to be set for extension loading. Rather use clangmajor instead. We need to rework the extension loading constraints in the future anyway

Developer checklist (address before review)

• [ ] Changelog.md updated
• [ ] Prepared update for depending repositories
• [ ] Documentation updated (public API changes only)
• [ ] API docstrings updated (public API changes only)
• [ ] Rebase → commit history clean
• [ ] Squash and merge → proper PR title