* use `pool.sks-keyservers.net` as keyserver as it's often more reliable
than `keyserver.ubuntu.com` as you can see with Google.
* put script to multiple lines
* build out-of-source
* use two CPUs
* make seems to fail if $(nproc) is used unlike WeeChat. ZNC has
same issue.
* mark build as failed if one job fails immediately without waiting for
all builds to finish.
* this doesn't stop jobs that didn't finish before build was marked
failed.
