Auto merge of #2710 - sbeckeriv:the-missing-lib, r=alexcrichton

Auto merge of #2710 - sbeckeriv:the-missing-lib, r=alexcrichton

The missing lib

Dearest Reviewer,

This pull request allows the value of project.links to be passed into
the build script via an environment variable. This closes #1220 . I have
added a test that makes sure that the environment variable is set. Also
note I am using CARGO_LIB not LIB because it appears to be used for
windows in appveyor. I think CARGO_LIB fits better.

I have also updated the documentation to reflect the new variable.

Thanks!
Becker