Preprocessor Horror

plaidCTF 2014 - g++ (re200)

For PlaidCTF2014, Eindbazen and fail0verflow joined forces as 0xffa, the Final Fail Alliance. Don't miss out on other write-ups at Eindbazen's site!
Although it seems like The Plague's projects are  open source, it's not
quite so simple to figure out what the source code  does. We believe
this project is supposed to print out secret information, but the KEY
variable in the Makefile has been lost. Find the key, build the
project, get us the information.

Oh noes, the key is gone!