Emit warning if rustc --print=native-static-libs reports a library dependency that has not been listed statically

[AndrewGaspar]

EDIT: I think compile-time derivation of the link requirements is intractible. It would force CMake to re-determine the graph at build time, which is almost certainly not possible. Instead, we should just emit a warning if we notice that rustc lists a static library dependency that has not been listed in the CMake file.

Things to do:

• Capture the stdlib static library dependencies on all supported platforms
• Add a NO_STD option (or derive this from the crate, if possible) to add_crate that opts out of linking against those static libraries
  • If Cargo allowed this to be specified in the TOML file, could be automatically derived
• Capture the results of rustc --print=native-static-libs

[AndrewGaspar]

See code here: https://github.com/AndrewGaspar/cmake-cargo/blob/master/src/main.rs#L200

[AndrewGaspar]

This probably isn't necessary since cargo_link_libraries is used to link local dependencies.