I tried to comment out all the things that were needed in order for these libraries to compile. So now they compile but obviously do not work.
Anyhow looking at these two commits you will see what would need to be re-implemented (which I have not done and I do not plan to do in the near future):
https://github.com/giuliomoro/Adafruit-GFX-Library/commit/452dc305f3ded58da66fdd1033d338c6d6e185c1
https://github.com/giuliomoro/Adafruit_SSD1306/commit/fb63b38e06bed5af5b1f9195b70d4e29f9e07206
Before going down this rabbit hole, though, I would encourage you to double check if there are any port of the libraries for the BeagleBone. Both projects have several hundred forks, I would be surprised if nothing is there already.