Some fun linker errors

This is a small repro of some linker errors we're seeing with rustc, Cargo, and the cc library.

The root of the problem is how rustc deduplicates redundant -l flags when calling the linker. This causes problems if we try to:

• link with multiple internal libraries ("A" and "B"), built using cc,
• each of which depends on an external library (in this case, libstdc++),
• but which depend on different sets of symbols from that library.

If one of the libraries depends on a strict subset of the symbols needed by the other, and the "smaller" library is linked after the "larger" one, then the compilation happens to succeed. If you link the "smaller" library before the "larger" one, then the compilation fails. (But only on Linux.)

Steps to reproduce

Verified on Linux:

1. git clone https://github.com/dcreager/rust-cc-linking
2. cd rust-cc-linking
3. cargo build -p cc-01-works-by-accident
4. cargo build -p cc-02-broken