Fixed painfully slow single process builds.

[jboero]

I've noticed the container builds are painfully slow as they don't enable parallel compilation. I always add the -j for defaulting to the number of available threads in the box which dramatically speeds up building. This would also be beneficial for the GitHub Actions and testing. Suggest this gets merged for automation sanity.

[slaren]

Worth keeping in mind that make -j does not default to the number of threads, but rather it removes any limits. Likely not an issue in most cases, but it can be very bad for systems/VMs with a very low number of processors.

[jboero]

Worth keeping in mind that make -j does not default to the number of threads

It's interesting because I've always heard that but in most Linux environments I've used lately it defaults to number of threads. Maybe it would be more helpful to

make -j$(nprocs) if the nprocs command is available?

[slaren]

I think that should work, but I don't know much about docker.

[giuseppe]

I think that should work, but I don't know much about docker.

nproc is cpuset aware so it works well from a container (i.e. if you limit the number of cpu cores a container can use, nproc will report the correct number of cores)