Switch to disable configuration through environment variables

It would be great if there were a switch to ignore environment variables to configure the build altogether.

Settings passed in via the environment become 'opt out', in the sense that if you want to be sure in a build script that a specific option is not set to something unexpected you have to override it on the command line. That makes it hard to create a hermetic build setup, since you never know what the developer's environment contains that might affect your build.