Allow users to limit number of cores using NJOBS envvar.

In the recent IHEP cluster, the number of processes are limit to 150. When a user builds the CEPCSW with all the cpu cores, the resources will be unavailable. So this MR allow users to reduce the number of cores:

$ NJOBS=12 ./build.sh

If users don't specify the number, then it is decided by the min(proc, nthreads/factor), where nthreads is ulimit -u and factor is 16.