[FEA] add parallel level option to ./build.sh

[wingkitlee0]

Is your feature request related to a problem? Please describe.
Hi, I am new to this project. The first thing I did was trying to build the library from scratch. As I have a small-memory laptop I kept getting OOM (cmake error 137) at about 33%.

It turns out there is already a PARALLEL_LEVEL env variable used in the script to pass to -j N option during cmake build..

Describe the solution you'd like
Add a CLI option (e.g., -j or --parallel) to ./build.sh

Describe alternatives you've considered
We can update the help message...but adding an option should be easy.

Additional context
I can make a PR for this if this makes sense.