According to Paul Doerwald:
Try 'touch'ing parse.c and parse.y, so that the two of them both get recompiled. Maybe that will help.
Indeed: `parse.c' and `parse.h' in `gpc-980511.tar.gz' are broken. Sorry for that. I am uploading a fixed archive now which differs from the previous one in that it expands to a subdirectory `gpc' rather than `gpc-980511'. (I hope this will not do any harm.;-)
Peter