Just to update:
It does not appear that the library itself even fits on the 84. I also get this error:
In file included from pcdtest.cpp:19:
C:\Users\Nick\Documents\Arduino\libraries\Adafruit_GFX/Adafruit_GFX.h:74: error: conflicting return type specified for 'virtual size_t Adafruit_GFX::write(uint8_t)'
C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/Print.h:71: error: overriding 'virtual void Print::write(uint8_t)'
In file included from C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/WProgram.h:17,
from C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/Arduino.h:4,
from C:\Users\Nick\Documents\Arduino\libraries\Adafruit_GFX/Adafruit_GFX.h:20,
from pcdtest.cpp:19:
C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/TinyDebugSerial.h:728: error: conflicting return type specified for 'virtual void TinyDebugSerial::write(uint8_t)'
C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/Print.h:71: error: overriding 'virtual size_t Print::write(uint8_t)'
In file included from C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/WProgram.h:18,
from C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/Arduino.h:4,
from C:\Users\Nick\Documents\Arduino\libraries\Adafruit_GFX/Adafruit_GFX.h:20,
from pcdtest.cpp:19:
C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/HardwareSerial.h:58: error: conflicting return type specified for 'virtual void HardwareSerial::write(uint8_t)'
C:\Users\Nick\Documents\Arduino\hardware\tiny\cores\tiny/Print.h:71: error: overriding 'virtual size_t Print::write(uint8_t)'
Eurgh, yeah, woops. I forgot that there are other bits in the core which are based around that version of Print being incorrect.
What you may have to do is to undo the changes to Print.h and Print.cpp, and instead change the Adafruit_GFX.h and Adafruit_GFX.cpp. Could you post the Adafruit library, and I can tell you what needs to be changed.
The other thing is though that the library seems to big for the 84. It compiles at almost 9kb. So it would need to be pruned in order to work. To display any text or anything it also requires this library:
I changed back the tiny cores, and added those changes to the GFX files and I still get the same errors but also with this at the end
In file included from pcdtest.cpp:19:
C:\Users\Nick\Documents\Arduino\libraries\Adafruit_GFX/Adafruit_GFX.h:27: error: virtual outside class declaration
I must be missing something. I have just tried compiling the example (stripped of all the line, box, circle, triangle stuff) and without modifying anything it compiles fine. Could you post your sketch?