Use gzip -n to prevent embedded buildtimes
This resolves a reproducible builds issue with build timestamps embedded in gzip metadata. It seems there's a second issue with file ordering (the content of usr/share/kdevappwizard/templates/plasma-wallpaper.tar.bz2
seems to depend on readdir), but this might be located in cmake itself.
https://reproducible.archlinux.org/api/v0/builds/51/diffoscope