A cycle between curl and curl-sys

I'm on debian, building from the debian-packaged rust curl crates.

In particular, i've got librust-curl-dev installed, and as you can see the local .cargo/config.toml points to the debian repo.

but with this project, when i cargo build, both curl and curl-sys get rebuilt each time.

I think this is due to curl getting rebuilt whenever curl-sys gets rebuilt, and curl-sys getting rebuilt whenever curl gets rebuilt (see alexcrichton/curl-rust#407)

0 dkg@alice:~/src/rust/curl-cycle$ cargo build
   Compiling curl-sys v0.4.49+curl-7.79.1
   Compiling curl v0.4.39
   Compiling curl-cycles v0.0.1 (/home/dkg/src/rust/curl-cycle)
    Finished dev [unoptimized + debuginfo] target(s) in 2.42s
0 dkg@alice:~/src/rust/curl-cycle$ cargo build
   Compiling curl-sys v0.4.49+curl-7.79.1
   Compiling curl v0.4.39
   Compiling curl-cycles v0.0.1 (/home/dkg/src/rust/curl-cycle)
    Finished dev [unoptimized + debuginfo] target(s) in 2.48s
0 dkg@alice:~/src/rust/curl-cycle$