Do not pass -jN to make, use cargo jobserver instead #92
Description
When compiling a crate that has jemallocator-sys as a dependency with cargo build -j4, I see a short spike where the number of cpu cores used is 8 instead of 4. The cause is this crate using the NUM_JOBS env var to call make, which ignores that cargo already has other running jobs, thus bringing the total number of running jobs to be higher than NUM_JOBS. This doesn't happen in any of the other *-sys crates, I guess because they use the cc crate with the parallel feature enabled instead of manually calling make.
The fix is to remove the -jN arg to make if the CARGO_MAKEFLAGS env var exists, and set MAKEFLAGS to CARGO_MAKEFLAGS instead.
I expect the implementation to be similar to this:
I guess using a crate to call make instead of doing it manually would handle this and other issues automatically, but I don't know what's the standard so I can't recommend any crates.