Switching GTK version

[Igor Korot <ikorot01 gmail com>]

 Hi, ALL,
I have setup to do my development for GTK+2 and GTK+3 on 2 different machines.
My project is on GitHub and there is one codebase.

What's the best way to set up Makefiles?

Is it feasible to define some environment variable and select GTK
version if it defined?

Or there is a better alternative?

Thank you.