-sys crates handling

[gyakovlev]

cardoe/cargo-ebuild#30
https://bugs.gentoo.org/709568

Some binding -sys crates are known to build private copy of a library by default, or if it's not found.
cargo-ebuild could maintain a list of know offenders and warn about extra dependencies, or even fill out ebuild.
also such ebuilds often need virtual/pkgconfig dep.

[lu-zero]

If system-deps is used for the -sys crates we could have more control through standardized env vars.

[thesamesam]

This would be really helpful for helping folks not familiar with packaging Rust in Gentoo, as I often have to share my list of workarounds from the various ebuilds right now.