Auto merge of #2847 - munyari:build_targets, r=alexcrichton

Warn about duplicated build targets

I basically wholesale copied `unique_names_in_targets` into a different `unique_build_targets` because of the different types. Is there any way I could have just used the existing functionality?

diff --cc src/cargo/util/toml.rs
Simple merge

diff --cc tests/build.rs
Simple merge