ghdl: broken if no backend available

[sgn]

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

[q66]

you should never conditionally define _package functions, and the "no backend" situation is irrelevant, since the llvm backend is available pretty much always, and even in case you did compile without a backend, you'd still get a "working" package (it would contain /usr/bin/ghdl which would tell you about no backend being available)