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

**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.