Allow linking to native library multiple times through build dependencies

Recently the mpi crate has stopped building because it transitively depends on different versions of the clang-sys crate which links to the native libclang. Here is the error message:

error: multiple packages link to native library `clang`, but a native library can be linked only once

package `clang-sys v0.22.0`
    ... which is depended on by `bindgen v0.33.2`
    ... which is depended on by `mpi-sys v0.1.1 (file:///Users/bsteinb/Documents/Programming/rust/rsmpi/mpi-sys)`
    ... which is depended on by `mpi v0.5.2 (file:///Users/bsteinb/Documents/Programming/rust/rsmpi)`
links to native library `clang`

package `clang-sys v0.21.2`
    ... which is depended on by `bindgen v0.31.3`
    ... which is depended on by `libffi-sys v0.6.0`
    ... which is depended on by `libffi v0.6.3`
    ... which is depended on by `mpi v0.5.2 (file:///Users/bsteinb/Documents/Programming/rust/rsmpi)`
also links to native library `clang`

I understand that this would be a problem if both versions of the clang-sys crate were to end up being linked to by the mpi crate, but they do not. bindgen (and transitively clang-sys) in both cases is a build dependency of mpi-sys and libffi-sys and is only linked to their respective build scripts and does not end up in the mpi crate. Should this really be an error, or is cargo being too cautious here?

Start with the mpi/mpi-sys dependency graph and the two clang-sys versions shown in the error, then trace Cargo's native-library link validation for build dependencies. Done means the behavior is decided and demonstrated with a regression case for this dependency graph.