build.sh sets with xz (was Re: vfs cache timing changes)

["Tom Spindler (moof)" <dogcow%babymeat.com@localhost>]

> PS: The xz compression for the debug set takes 36 minutes on my machine.
> We shoudl do something about it. Matt to use -T for more parallelism?

On older machines, xz's default settings are pretty much unusable,
and USE_XZ_SETS=no (or USE_PIGZGZIP=yes) is almost a requirement.
On my not-exactly-slow i7 6700K, build.sh -j4 parallel is just fine
until it hits the xz stage; gzip is many orders of magnitude faster.
Maybe if xz were cranked down to -2 or -3 it'd be better at not
that much of a compression loss, or it defaulted to the higher
compression level only when doing a `build.sh release`.