Fixing number of cores detection for boost builds

This way of cores detection:
```bash
cat /proc/cpuinfo | grep "cpu cores" | uniq | awk '{print $NF}'
```
is cumbersome and didn't work in some cases, e.g. for VM on VirtualBox with one CPU.
It's much better to use something else, e.g.
```bash
getconf _NPROCESSORS_ONLN
```
as stated in
https://stackoverflow.com/questions/6481005/obtain-the-number-of-cpus-cores-in-linux
```nproc``` also works well but didn't compatible with MacOS X.