Auto merge of #4125 - alexcrichton:rerun-if-env-changed, r=matklad

Auto merge of #4125 - alexcrichton:rerun-if-env-changed, r=matklad

Implement `cargo:rerun-if-env-changed=FOO`

This commit implements a new method of rerunning a build script if an
environment variable changes. Environment variables are one of the primary
methods of giving inputs to a build script today, and this'll help situations
where if you change an env var you don't have to remember to clean out an old
build directory to ensure fresh results.

Closes #2776