The missing lib

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

[Updated]
Github comments moved to CARGO_MANIFEST_LINKS
Fix if statement