message: Document the CARGO_CFG_* environment variables - fixes
diff: @@ -80,6 +80,15 @@ let out_dir = env::var("OUT_DIR").unwrap();
  built, this environment variable will be present
  where `<name>` is the name of the feature uppercased
  and having `-` translated to `_`.
  +* `CARGO_CFG_<cfg>` - For each [configuration option][configuration] of the
  + package being built, this environment variable will
  + contain the value of the configuration, where `<cfg>` is
  + the name of the configuration uppercased and having `-`
  + translated to `_`.
  + Boolean configurations are present if they are set, and
  + not present otherwise.
  + Configurations with multiple values are joined to a
  + single variable with the values delimited by `,`.
  * `OUT_DIR` - the folder in which all output should be placed. This folder is
  inside the build directory for the package being built, and it is
  unique for the package in question.
  @@ -107,6 +116,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
  [links]: build-script.html#the-links-manifest-key
  [profile]: manifest.html#the-profile-sections
  +[configuration]: https://doc.rust-lang.org/reference/attributes.html#conditional-compilation
  [clang]:http://clang.llvm.org/docs/CrossCompilation.html#target-triple
  [jobserver]: http://make.mad-scientist.net/papers/jobserver-implementation/
  