Get outside config value inside of Constants?

I want to setup my constants.php file variables to be different depending whether on dev site or live site. How can I grab a variable from either config.php or another file so I can upload the same updated constants file to both servers.

In other words, I'd like to do something like this in Constants.php

if (DEVELOPMENT === true) {
 //define these CONSTANTS
} else {
 //define these CONSTANTS
}

But i'd like to grab the DEVELOPMENT value from another file, so I don't have to worry about uploading the same constants.php file to both my hosting situations.