xios: update to v2.6 (#48680)

* xios: update to v2.6

- xios: update to v2.6
- xios: patch remap earcut.hpp for missing include file
- xios: address C++11 requirement
- xios: instruct to use external boost and blitz libraries

* xios: bump revision to 2714

- Bump revision to changeset 2714
- Deprecate older versions that I can't manage to compile
- Confirm clang workaround is still needed