Conditional environment variables?

Is there a way to define environment variables that get set based on a build target environment? For example, I have modified Blur Admin to consume data from a REST API. I need to use a different API URL for development, staging, and production.

My hacked-together approach involves defining the URL as a window property and then manually changing the URL based on which environment I'm running/building for.

I'm wondering whether a mechanism already exists in Blur Admin for dealing with this in a better way. If not, I'm wondering what solutions others have come up with and whether it would make sense to incorporate something into the project.