This is because it is looking for Kernel Sources. I attempted to download said program to my build machine but could only find the rt73* package. Anyway, I download that package, tried a make and it still would not build. After modifying the Makefile to point to my /usr/src/18.104.22.168 sources, I was finally able to get it to compile.
Needless to say, you state that you are using BT3 which is officially not supported, but that is what you are going to need to do in order to get it to compile, you will need kernel sources.
If anyone else has any easier way, please feel free to help the OP