How to Enable Features When Building ArduPlane from Source?

I have a forked and modified version of the ArduPlane source code. I’m building it using:

./waf configure #...
./waf plane

However, I need to enable specific features, such as MAVLink GPS. I know that most users achieve this by using the Custom Firmware Builder, which allows selecting features easily. Unfortunately, since I have custom modifications, I can’t use that tool.

I haven’t been able to find clear documentation on how to enable specific features using waf. How can I achieve the same feature selection manually while building from source?

Any help would be greatly appreciated!