Update Eigen cmake header references

[ghutchis]

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[github-actions]

Here are the build results
Avogadro2.AppImage
Win64.exe
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[ghutchis]

@drew-parsons - seems to be failing on the Windows build for Python wheels, which is annoying. It's looking for -DEigen3_INCLUDE_DIR:PATH=C:/eigen but I can't figure out where that happens.

[drew-parsons]

What is the location of the Windows installation exactly?

Does the suggestion at https://stackoverflow.com/a/67303140/12401525 help, setting EIGEN3_INCLUDE_DIR (or Eigen3_DIR) when calling cmake in the windows script? Is %EIGEN3_ROOT% defined for windows?

[**edit Eigen docs suggest setting Eigen3_DIR if it's installed in a non-default path]

[drew-parsons]

There's also https://stackoverflow.com/a/55476486/12401525
which suggests the Windows location might be C:\Program Files(X86)\Eigen3

[avo-bot]

This pull request has been mentioned on Avogadro Discussion. There might be relevant details there:

https://discuss.avogadro.cc/t/december-2023-live-updates/5051/1

[github-actions]

Here are the build results
Avogadro2.AppImage
Artifacts will only be retained for 90 days.