How to pass -J switch to compiler via DUB?

[Andrey via Digitalmars-d-learn]

Hi,
How to pass -J switch to compiler via DUB?
I want to import some text file at compile time:

string data = import("vertex.glsl");

In dub.json:
"dflags": [
    "-J=vertex.glsl"
]

The file itself is located on the same level as "dub.json".