Parallel feature gives no benefit since jobserver requires libc

[RCasatta]

I wanted to share my experience in trying to improve rust-bitcoin crate build times.
I noticed secp256k1-sys native build script was taking a significant amount of time

And I noticed cc wasn't using the parallel feature, so I enabled it. This successfully shaved a second from secp256k1-sys build time, however, the overall build time is worst because now cc has now to wait libc to start building.

I wonder if it's possible for jobserver to avoid to depend on libc

[NobodyXu]

Though it's possible for cc to vendor jobserver since it only uses a subset of its API and already vendored windows-sys APIs and have function for making anonymous pipe on Unix and Windows.

Originally posted on rust-lang/jobserver-rs#44 (comment)

[NobodyXu]

libc does a lot of hard work of defining APIs for different targets based on cfg.

Since it only takes ~1s to compile, it doesn't worth the time to vendor it.