nproc is a standard util that prints amount of available processes.
-j is a flag that specifies how many cores can be utilized by `make` (default is 1)
-j$(nproc) allows to use all the available machine cores, potentially significantly speeding up completion of CI actions
* Also pass `$*` to `configure` when invoking `ci-build.sh`, so one can
e.g. run `./ci-build.sh --without-xscreensaver`
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>