Auto merge of #3951 - alexcrichton:lift-cdylib, r=alexcrichton

Don't use `-C metadata` cdylibs like we do with dylibs

Dylibs don't get any metadata/extra filename info applied to them as "final
targets" because it can mess with system-specific information (e.g. on OSX) so
this just applies the same logic to cdylibs which need similar treatment on more
platforms (like Windows).

Closes #3934

diff --cc src/cargo/core/manifest.rs
Simple merge

diff --cc tests/build.rs
Simple merge