Auto merge of #1629 - alexcrichton:env-rustc, r=alexcrichton

These commits add two vectors through which the compiler run can be configured:

* A `RUSTC` environment variable
* A `build.rustc` configuration variable in a `.cargo/config` file

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

diff --cc tests/test_cargo_compile.rs
Simple merge