Hi @Stefan,
First of all I have to ask: do you need to rebuild GRBL ? Just curious what you are trying to do with those two lines.
Officially I should state that if you don’t run the stock firmware, support may not be able to assist you.
Now if you have good reasons to rebuild GRBL, those instructions should get you there:
Note that the stock Shapeoko firmware has a few customizations, custom config.h linked here
At which step does it fail and what does it say ?
I have done this a few times successfully.