correct syntax in maxima's spkg-configure.m4

[dimpase]

remove $ to print the version correctly, and remove unneeded "..."

the extra $ leads to the minimal version not being printed, and extra "..." around minimal version are confusing (and not needed)

📝 Checklist

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⌛ Dependencies

[dimpase]

I discovered all this while figuring out why the trunk maxima (which is version 5.47foo by default) doesn't get recognised. Perhaps we should add a comment on this.

[github-actions]

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