xz supports parallel compression which obviously speeds up the time taken to run the 'dist-xz' rule, but it also speeds up *decompression* too, as the parallel compressor creates different output which can be decompressed in parallel.
There's two options, I think, for automake: 1. Pass -T+1 to xz if xz is >= 5.4.0 (a runtime check would be needed, but it's cheap to do) as this uses 1 thread, but uses the parallel compressor to do it, so allows decompression in parallel by users; 2. Pass -T0 (or -Tn, n >= 1) to allow autodetection of the number of available CPU cores (or n if specified) which also then naturally uses the threaded compressor.
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