Do not pass -jN to make, use cargo jobserver instead

[tmpolaczyk]

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:

https://github.com/rust-lang/cmake-rs/blob/c4a60dd154dd90e469dffc41a1faa717704f90b3/src/lib.rs#L830

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.

[BusyJay]

The fix is to remove the -jN arg to make if the CARGO_MAKEFLAGS env var exists, and set MAKEFLAGS to CARGO_MAKEFLAGS instead.

Make sense. Would you like to send a PR?

[tmpolaczyk]

I tried to run cargo build but it gave me a strange error "configure: error: cannot find sources (Makefile.in)", and I didn't find any documentation on what should I do to fix it, so I will let some other contributor implement it.

[BusyJay]

Likely the same problem as #32.