I have put a new gpc snapshot at:
http://www.math.uni.wroc.pl/~hebisch/gpc/gpc-20051116.tar.bz2
This snapshot has integrated preprocessor. This should solve problems with (lack of) predefined symbols when using 3.3.x and 3.4.x backends. The preprocessor is just integrated -- inside it is the same preprocessor as before. There is new `--preprocessed' option, but the change is intended to be as compatible as possible. Still, the code to handle command line changed, so there may be some unintended change. Also, there is no gpcpp executable anymore (use gpc -E as preprocessor).
There are also minor fixes, see:
http://www.math.uni.wroc.pl/~hebisch/gpc/NEWS-20051116
and
http://www.math.uni.wroc.pl/~hebisch/gpc/Fixed-20051116