Improved 'make' commandline option

[ntolazzi]

The former call 'make -j' keeps spawning an unlimited amount of threads. This reproducibly leads to memory exhaustion on my system (Manjaro Linux with 32GB RAM) and a subsequent crash. 'make -j $(nproc)' only spawns a limited number of compilations threads adjusted to the available cores.

[linux-foundation-easycla[bot]]

• :x: - login: @ntolazzi / name: Nicolas Tolazzi . The commit (81da1c9533fd52b6c16caefc429bfd9f97e293a5) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[XuanWang-Amos]

Thanks for the change, can you please sign the CLA? We'll start review after that. Also, this looks like duplicated with https://github.com/grpc/grpc.io/pull/983