There's a closed pull request on the void-packages repository

ghdl: broken if no backend available
https://github.com/void-linux/void-packages/pull/28311

Description:
While we're at it, let xbps-src figure out which subpackages will be
built. I don't like `unset`.

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@q66 feel free to revert the `subpackages` calculation part. Beside my dislike to unset, I think conditional definition of subpackages can reduce one warning :-p