How to set environment variables for build

[Mayuresh <mayuresh%acm.org@localhost>]

I have seen examples of how to set environment variables if we are writing
a command for a specific make target.

However, for example in wip/py-dd (See TODO) I need to set the environment
variables without having to override any command.

What is the right way to specify such environment variables globally for a
build?

-- 
Mayuresh