Require gperf at build-time

[brndnmtthws]

Rather than allowing the build to continue without gperf, we should fail with an error so that the colour behaviour does not change in a backward incompatible manner. The old colour behaviour should continue to work going forward.

This resolves #1868.

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[Caellian]

Looks okay for the intent of the PR.

I'd personally add a build option to disable color name parsing (and gperf build dep) as well - I don't use color names because hex codes are supported and I believe a lot of others don't either, so it might be a good way to reduce binary size (both previous impl and gperf take up sizeable amount of space in .text).

[brndnmtthws]

I'd personally add a build option to disable color name parsing (and gperf build dep) as well - I don't use color names because hex codes are supported and I believe a lot of others don't either, so it might be a good way to reduce binary size (both previous impl and gperf take up sizeable amount of space in .text).

That's reasonable, just need to maintain backward compatibility.