Tracking how *-sys crates vendor sources

[weihanglo]

Like #7 but for vendoring stuff.

By having a better understanding of how they vendor libraries, we could come up with a common declarative interface covering most use cases.

[weihanglo]

• curl-sys
  • Git submodule
• libgit2-sys
  • Git submodule
• openssl-sys
  • crate source (openssl-src, which imports openssl as Git submodule)
• libssh2-sys
  • Git submodule
• zstd-sys
  • Git submodule
• lzma-sys
  • Git submodule though build.rs won't clone for you.
• libz-sys
  • Git submodule though build.rs won't clone for yoiu.